Qualified Employee Benefit Plans - Net (Benefit) Expense Under the Retirement Plan (Details) - Retirement Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost on projected benefit obligations	$ 4,999	$ 4,972	$ 4,816
Expected return on plan assets	(5,261)	(5,407)	(6,176)
Amortization of prior service cost	24	24	0
Recognized actuarial loss	1,097	959	979
Net pension (benefit) expense	$ 859	$ 548	$ (381)