Employee Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit cost
Recognized net actuarial gain due to curtailments	$ 0	$ 302	$ 0
DB Pension Plan
Components of net periodic benefit cost
Service cost	0	0	0
Interest cost	2,371	2,936	2,508
Expected return on plan assets	(5,416)	(4,933)	(5,648)
Amortization of prior service (credit) cost	(10)	(10)	(10)
Recognized net actuarial loss	1,411	1,334	1,118
Recognized net actuarial gain due to curtailments	0	0	0
Net periodic benefit (credit) cost	(1,644)	(673)	(2,032)
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial loss (gain)	3,899	2,776	7,761
Recognized actuarial loss	(1,411)	(1,334)	(1,118)
Prior service credit	0	0	0
Recognized prior service cost (credit)	10	10	10
Prior service cost (credit) recognized due to curtailment	0	0	0
Recognized in other comprehensive income (loss)	2,498	1,452	6,653
Total recognized in net periodic benefit cost and other comprehensive income (loss)	854	779	4,621
Life and Healthcare Plan
Components of net periodic benefit cost
Service cost	173	159	212
Interest cost	245	289	270
Expected return on plan assets	0	0	0
Amortization of prior service (credit) cost	(61)	(62)	(62)
Recognized net actuarial loss	155	0	62
Recognized net actuarial gain due to curtailments	0	(399)	0
Net periodic benefit (credit) cost	512	(13)	482
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial loss (gain)	1,340	1,218	(1,337)
Recognized actuarial loss	(155)	0	(62)
Prior service credit	0	(203)	0
Recognized prior service cost (credit)	61	62	62
Prior service cost (credit) recognized due to curtailment	0	399	0
Recognized in other comprehensive income (loss)	1,246	1,476	(1,337)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	1,758	1,463	(855)
SERP Plan
Components of net periodic benefit cost
Service cost	214	157	160
Interest cost	914	909	833
Expected return on plan assets	0	0	0
Amortization of prior service (credit) cost	285	104	87
Recognized net actuarial loss	800	343	539
Recognized net actuarial gain due to curtailments	0	0	0
Net periodic benefit (credit) cost	2,213	1,513	1,619
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial loss (gain)	4,070	6,128	(2,987)
Recognized actuarial loss	(800)	(343)	(539)
Prior service credit	0	2,022	0
Recognized prior service cost (credit)	(285)	(104)	(87)
Prior service cost (credit) recognized due to curtailment	0	0	0
Recognized in other comprehensive income (loss)	2,985	7,703	(3,613)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 5,198	$ 9,216	$ (1,994)