Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 2,178	$ 1,701	$ 4,356	$ 3,402
Interest cost	3,297	3,316	6,594	6,664
Expected return on plan assets	(3,696)	(4,553)	(7,392)	(9,106)
Amortization of prior service cost (credit)	169	61	338	122
Amortization of net actuarial loss	1,865	550	3,730	1,192
Settlement loss	0	843	0	1,627
Net periodic benefit cost	3,813	1,918	7,626	3,901
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	1,875	1,944	3,750	3,888
Interest cost	6,351	7,726	12,702	15,452
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(1,552)	307	(3,104)	614
Amortization of net actuarial loss	5,659	3,892	11,318	7,784
Settlement loss	0	0	0	0
Net periodic benefit cost	$ 12,333	$ 13,869	$ 24,666	$ 27,738