Postretirement Benefits (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 736,705	$ 557,279
Service cost for benefits earned	30,433	22,510	$ 26,896
Interest cost on benefit obligation	30,755	28,112	25,474
Plan amendments	0	164
Actuarial (gain) loss	(60,774)	139,816
Benefits paid	(12,539)	(11,176)
Projected benefit obligation, end of year	724,580	736,705	$ 557,279
Accumulated benefit obligation, end of year	$ 583,432	$ 583,469