Pension Plan and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 20,600	$ 22,700
Estimated cash contributions in the current fiscal year			28,300
Components of the pension plans and other postretirement benefits
Service cost	$ 5,594	$ 4,715	15,728	15,980
Interest cost	5,764	5,072	16,670	15,078
Expected return on plan assets	(5,462)	(4,770)	(16,370)	(14,354)
Amortization of prior service cost	1,555	1,502	4,664	4,506
Recognized net actuarial loss	1,743	2,308	4,329	7,108
Net periodic benefit cost	9,194	8,827	25,021	28,318
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			6,700	7,500
Estimated cash contributions in the current fiscal year			9,600
Components of the pension plans and other postretirement benefits
Service cost	1,045	3,043	5,653	8,044
Interest cost	625	2,101	4,225	5,328
Expected return on plan assets	(1,005)	(908)	(3,097)	(2,660)
Amortization of prior service cost	10	12	32	34
Recognized net actuarial loss	(482)	2,182	2,041	5,099
Net periodic benefit cost	193	$ 6,430	8,854	$ 15,845
Actuarial decrease due to reduction in retiree medical claims cost	$ 6,200		$ 7,000