Postretirement Benefits (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 816,659	$ 724,580
Service cost for benefits earned	31,106	28,201	$ 30,433
Interest cost on benefit obligation	34,275	33,125	30,755
Plan amendments	5,050	2,114
Actuarial loss	82,940	43,032
Benefits paid	(16,184)	(14,393)
Settlement cost	(2,180)
Projected benefit obligation, end of year	951,666	816,659	$ 724,580
Accumulated benefit obligation, end of year	$ 767,527	$ 657,969