Employee Benefit Plans - Expense Components of Pension Plan and SERP (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 3,318	$ 3,133	$ 2,940
Expected return on plan assets, net of expenses	(3,969)	(3,851)	(3,876)
Amortization of net loss	3,040	2,256	2,095
Special termination benefits			860
Settlement charges	1,409		3,345
Total	3,798	1,538	5,364
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,904	1,519	1,713
Amortization of net loss	3,171	2,471	2,354
Special termination benefits			290
Settlement charges	2,006	2,514	1,121
Total	$ 7,081	$ 6,504	$ 5,478