Annual report pursuant to Section 13 and 15(d)

Accumulated Other Comprehensive Income (Accumulated Other Comprehensive Income, Net of Related Tax Effects) (Details)

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Accumulated Other Comprehensive Income (Accumulated Other Comprehensive Income, Net of Related Tax Effects) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Accumulated other comprehensive income at beginning of period $ (22,617) $ (20,836) $ 15,833
Other comprehensive income (loss) before reclassifications 15,660 (2,047) (23,970)
Amounts reclassified from other comprehensive income (1,930) 266 (12,699)
Net other comprehensive income (loss) during the period 13,730 (1,781) (36,669)
Accumulated other comprehensive income at end of period (8,887) (22,617) (20,836)
Unrealized Holding Gains (Losses) on Available-For-Sale Securities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Accumulated other comprehensive income at beginning of period (4) (2) 4
Other comprehensive income (loss) before reclassifications 0 (2) (6)
Amounts reclassified from other comprehensive income 0 0 0
Net other comprehensive income (loss) during the period 0 (2) (6)
Accumulated other comprehensive income at end of period (4) (4) (2)
Unrealized Holding Gains (Losses) on Derivative Transactions [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Accumulated other comprehensive income at beginning of period 6,773 6,087 8,995
Other comprehensive income (loss) before reclassifications (15,559) 420 9,791
Amounts reclassified from other comprehensive income (1,930) 266 (12,699)
Net other comprehensive income (loss) during the period (17,489) 686 (2,908)
Accumulated other comprehensive income at end of period (10,716) 6,773 6,087
Foreign Currency Translation Adjustment [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Accumulated other comprehensive income at beginning of period (29,386) (26,921) 6,834
Other comprehensive income (loss) before reclassifications 31,219 (2,465) (33,755)
Amounts reclassified from other comprehensive income 0 0 0
Net other comprehensive income (loss) during the period 31,219 (2,465) (33,755)
Accumulated other comprehensive income at end of period $ 1,833 $ (29,386) $ (26,921)