Pension Plan and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 14,000	$ 14,100
Estimated cash contributions in the current fiscal year			27,300
Components of the pension plans and other postretirement benefits
Service cost	$ 5,067	$ 5,632	10,134	11,265
Interest cost	5,453	5,003	10,906	10,006
Expected return on plan assets	(5,454)	(4,792)	(10,908)	(9,584)
Amortization of prior service cost	1,555	1,502	3,109	3,004
Recognized net actuarial loss	1,293	2,400	2,586	4,800
Net periodic benefit cost	7,914	9,745	15,827	19,491
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			3,300	3,700
Estimated cash contributions in the current fiscal year			13,400
Components of the pension plans and other postretirement benefits
Service cost	2,304	2,500	4,608	5,001
Interest cost	1,800	1,614	3,600	3,227
Expected return on plan assets	(1,046)	(876)	(2,092)	(1,752)
Amortization of prior service cost	11	11	22	22
Recognized net actuarial loss	1,261	1,458	2,523	2,917
Net periodic benefit cost	$ 4,330	$ 4,707	$ 8,661	$ 9,415