Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 2,178	$ 1,701	$ 6,534	$ 5,103
Interest cost	3,297	3,316	9,891	9,980
Expected return on plan assets	(3,696)	(4,553)	(11,088)	(13,659)
Amortization of prior service cost (credit)	169	61	507	183
Amortization of net actuarial loss	1,865	550	5,595	1,742
Settlement loss			0	1,627
Net periodic benefit cost	3,813	1,075	11,439	4,976
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	1,875	1,944	5,625	5,832
Interest cost	6,351	7,726	19,053	23,178
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(1,552)	307	(4,656)	921
Amortization of net actuarial loss	5,659	3,892	16,977	11,676
Settlement loss			0	0
Net periodic benefit cost	$ 12,333	$ 13,869	$ 36,999	$ 41,607