Employee Benefit Plan - Pre-Tax Amounts Recognized as Component of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
DB Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	$ 39,960	$ 39,601	$ 38,695
Prior service cost (credit)	(30)	(40)	(55)
Total	39,930	39,561	38,640
Actuarial loss	1,041
Prior service cost	(10)
Total	1,031
Life and Healthcare Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	1,767	1,093	889
Prior service cost (credit)	(668)	235	250
Total	1,099	1,328	1,139
Actuarial loss	62
Prior service cost	(62)
Total	0
SERPs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	9,086	7,077	6,739
Prior service cost (credit)	602	689	576
Total	9,688	$ 7,766	$ 7,315
Actuarial loss	529
Prior service cost	87
Total	$ 616