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, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 116,233	$ 125,200
Interest cost	4,944	4,771	$ 4,999
Actuarial loss (gain)	20,411	(9,918)
Benefits paid	(5,475)	(3,820)
Projected benefit obligation at end of year	136,113	116,233	125,200
Change in plan assets:
Plan assets at fair value at beginning of year	98,584	100,706
Actual return on plan assets	16,971	(3,302)
Employer contribution	4,000	5,000
Benefits paid	(5,475)	(3,820)
Plan assets at fair value at end of year	114,080	98,584	$ 100,706
Funded status	$ (22,033)	$ (17,649)