Qualified Employee Benefit Plans - The Retirement Plan's Projected Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - Retirement Plan - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 136,113,000	$ 116,233,000
Interest cost	4,443,000	4,944,000	$ 4,771,000
Actuarial loss (gain)	16,131,000	20,411,000
Benefits paid	(5,563,000)	(5,475,000)
Projected benefit obligation at end of year	151,124,000	136,113,000	116,233,000
Change in plan assets:
Plan assets at fair value at beginning of year	114,080,000	98,584,000
Actual return on plan assets	16,505,000	16,971,000
Employer contribution	0	4,000,000
Benefits paid	(5,563,000)	(5,475,000)
Plan assets at fair value at end of year	125,022,000	114,080,000	$ 98,584,000
Funded status	$ (26,102,000)	$ (22,033,000)