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Benefit Plans - Changes to Benefit Obligations and Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Funded status and amounts recognized in balance sheets
Pension liabilities for underfunded plans				$ (143,373)	$ (138,222)
Pension Benefits
Changes to PBO or APBO
Benefit obligation at beginning of year	$ 213,285	$ 199,072
Service cost		14
Interest cost	6,519	8,261
Actuarial loss (gain)	14,935	22,573
Curtailment gain		(1,061)
Benefit payments	(15,347)	(15,574)
Benefit obligation at end of year	219,392	213,285	$ 199,072
Changes to fair value of plan assets
Fair value of plan assets at beginning of year	112,659	99,581
Actual return on plan assets	9,637	20,863
Benefit payments from trust	(7,501)	(7,785)
Fair value of plan assets at end of year	114,795	112,659	99,581
Funded status and amounts recognized in balance sheets
Pension assets for overfunded plans				16,237	16,291
Pension liabilities for underfunded plans				(120,834)	(116,917)
Funded status				(104,597)	(100,626)
PBO, ABO and fair value of plan assets
Projected benefit obligation	213,285	213,285	199,072	219,392	213,285
Accumulated benefit obligation				219,324	213,202
Fair value of plan assets	114,795	99,581	99,581	114,795	112,659
Overfunded (underfunded) portion of PBO/ABO				(104,597)	(100,626)
Pension Benefits \| Funded Plan
Changes to PBO or APBO
Benefit obligation at beginning of year	96,368
Interest cost	2,946	3,808	3,420
Benefit obligation at end of year	98,558	96,368
Changes to fair value of plan assets
Fair value of plan assets at beginning of year	112,659
Fair value of plan assets at end of year	114,795	112,659
PBO, ABO and fair value of plan assets
Projected benefit obligation	96,368	96,368		98,558	96,368
Accumulated benefit obligation				98,558	96,368
Fair value of plan assets	112,659	112,659		114,795	112,659
Overfunded (underfunded) portion of PBO/ABO				16,237	16,291
Pension Benefits \| Unfunded Plan
Changes to PBO or APBO
Benefit obligation at beginning of year	116,917
Benefit obligation at end of year	120,834	116,917
PBO, ABO and fair value of plan assets
Projected benefit obligation	116,917	116,917		120,834	116,917
Accumulated benefit obligation				120,766	116,834
Overfunded (underfunded) portion of PBO/ABO				(120,834)	(116,917)
Other Benefits
Changes to PBO or APBO
Benefit obligation at beginning of year	21,305	19,716
Service cost	768	710
Interest cost	640	808
Actuarial loss (gain)	303	540
Benefit payments	(478)	(469)
Benefit obligation at end of year	22,538	21,305	19,716
Funded status and amounts recognized in balance sheets
Pension liabilities for underfunded plans				(22,538)	(21,305)
Funded status				(22,538)	(21,305)
PBO, ABO and fair value of plan assets
Projected benefit obligation	$ 21,305	$ 21,305	$ 19,716	$ 22,538	$ 21,305