Qualified Employee Benefit Plans - The Retirement Plan's Projected Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - Retirement Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 111,315	$ 107,784
Interest cost	4,999	4,972	$ 4,816
Actuarial loss (gain)	12,617	1,794
Benefits paid	(3,731)	(3,235)
Projected benefit obligation at end of year	125,200	111,315	107,784
Change in plan assets:
Plan assets at fair value at beginning of year	86,699	86,292
Actual return on plan assets	13,738	3,642
Employer contribution	4,000	0
Benefits paid	(3,731)	(3,235)
Plan assets at fair value at end of year	100,706	86,699	$ 86,292
Funded status	$ (24,494)	$ (24,616)