Qualified Employee Benefit Plans - The Retirement Plan's Projected Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - Retirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 125,200	$ 111,315
Interest cost	4,771	4,999	$ 4,972
Actuarial (gain) loss	(9,918)	12,617
Benefits paid	(3,820)	(3,731)
Projected benefit obligation at end of year	116,233	125,200	111,315
Change in plan assets:
Plan assets at fair value at beginning of year	100,706	86,699
Actual return on plan assets	(3,302)	13,738
Employer contribution	5,000	4,000
Benefits paid	(3,820)	(3,731)
Plan assets at fair value at end of year	98,584	100,706	$ 86,699
Funded status	$ (17,649)	$ (24,494)