Annual report pursuant to Section 13 and 15(d)

Retirement Savings Plans (Narrative) (Details)

v3.8.0.1
Retirement Savings Plans (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Liability to participants under deferred compensation plan $ 9,319,000 $ 8,411,000  
Current liability to participants under deferred compensation plan 1,437,000 472,000  
401(k) Profit-Sharing Plan [Member]      
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Company contributions to the plan 7,666,000 7,754,000 $ 6,981,000
Deferred Compensation Plan [Member]      
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Company contributions to the plan 210,000 200,000 $ 180,000
Other Noncurrent Assets [Member] | Deferred Compensation Plan [Member]      
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Fair value of mutual fund investments 9,319,000 8,411,000  
Short-term Investments [Member] | Deferred Compensation Plan [Member]      
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Fair value of mutual fund investments $ 1,437,000 $ 472,000