Pension and Retirement Plans, Deferred Compensation Arrangement with Individual, Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Company's 401(K) contributions for the year	$ 7,182	$ 5,943	$ 5,292
SERP [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Income expense due to change in the fair market value of Company stock held in the SERP	575	(1,742)	$ (241)
Cost [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	6,549	5,432
Cost [Member] | Company Stock [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	1,960	1,958
Cost [Member] | Equity Securities [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	4,589	3,474
Market [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	8,552	7,882
Market [Member] | Company Stock [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	3,589	4,455
Market [Member] | Equity Securities [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plan Assets	$ 4,963	$ 3,427