Postretirement Benefits (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 724,580	$ 736,705
Service cost for benefits earned	28,201	30,433	$ 22,510
Interest cost on benefit obligation	33,125	30,755	28,112
Plan amendments	2,114	0
Actuarial loss (gain)	43,032	(60,774)
Benefits paid	(14,393)	(12,539)
Projected benefit obligation, end of year	816,659	724,580	$ 736,705
Accumulated benefit obligation, end of year	$ 657,969	$ 583,432