Postretirement Benefits - Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 886,165	$ 951,666
Service cost for benefits earned	33,854	38,052	$ 31,106
Interest cost on benefit obligation	39,306	35,382	34,275
Plan amendments	452	3,007
Actuarial loss (gain)	138,144	(123,910)
Benefits paid	(43,454)	(18,032)
Projected benefit obligation, end of year	1,054,467	886,165	$ 951,666
Accumulated benefit obligation, end of year	858,209	$ 727,340
Projected benefit obligations increase	$ 168,300
Discount rate used to measure future benefit obligations (percentage)	3.59%	4.47%
Increase in benefits paid due to offering a one-time lump sum payment to former vested employees	$ 25,400