Benefit Plans - Change in Net Actuarial Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Benefit Plans
Percentage of greater of projected benefit obligation or fair value of plan assets for amortization of actuarial gains or losses	5.00%
Amortization period for actuarial gains and losses that exceed threshold	5 years
Pension Benefits
Change in net actuarial loss and amortization
Net actuarial loss (gain) at beginning of year	$ 40,061	$ 41,822
Amortization cost	(5,806)	(6,995)	$ (7,315)
Liability loss (gain)	14,935	21,512	(6,619)
Asset (gain) loss	(4,839)	(16,278)	12,209
Net actuarial loss (gain) at end of year	44,351	40,061	41,822
Other Benefits
Change in net actuarial loss and amortization
Net actuarial loss (gain) at beginning of year	(1,582)	(2,507)
Amortization cost	211	385
Liability loss (gain)	303	540	(2,755)
Net actuarial loss (gain) at end of year	$ (1,068)	$ (1,582)	$ (2,507)