Postretirement Benefits - Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 951,666	$ 816,659
Service cost for benefits earned	38,052	31,106	$ 28,201
Interest cost on benefit obligation	35,382	34,275	33,125
Plan amendments	3,007	5,050
Actuarial (gain) loss	(123,910)	82,940
Benefits paid	(18,032)	(16,184)
Settlement cost	0	(2,180)
Projected benefit obligation, end of year	886,165	951,666	$ 816,659
Accumulated benefit obligation, end of year	727,340	$ 767,527
Projected benefit obligations decrease	$ 65,500
Discount rate used to measure future benefit obligations	4.47%	3.73%