Postretirement Benefits - Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 1,054,467	$ 886,165
Service cost for benefits earned	43,492	33,854	$ 38,052
Interest cost on benefit obligation	37,578	39,306	35,382
Plan amendments	0	452
Actuarial loss	134,470	138,144
Benefits paid	(23,848)	(43,454)
Projected benefit obligation, end of year	1,246,159	1,054,467	$ 886,165
Accumulated benefit obligation, end of year	1,006,884	$ 858,209
Projected benefit obligations increase	$ 191,700
Discount rate used to measure future benefit obligations	2.96%	3.59%
Decrease in benefits paid due to offering a one-time lump sum payment to former vested employees	$ 19,600