Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 13,448	$ 14,907	$ 40,344	$ 44,721
Interest cost on benefit obligation	32,912	30,996	98,735	92,987
Expected return on plan assets	(43,477)	(44,808)	(130,429)	(134,424)
Amortization of:
Prior service cost	132	148	395	446
Net actuarial loss	10,179	7,764	30,538	23,292
Net periodic benefit cost	13,194	9,007	39,583	27,022
Portion of cost charged to expense	6,476	4,433	19,427	15,653
Other postretirement benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	3,748	4,207	11,245	12,620
Interest cost on benefit obligation	7,430	7,026	22,291	21,077
Expected return on plan assets	(9,123)	(9,214)	(27,371)	(27,641)
Amortization of:
Prior service cost	(9,471)	(9,492)	(28,413)	(28,476)
Net actuarial loss	1,147	1,220	3,442	3,661
Net periodic benefit cost	(6,269)	(6,253)	(18,806)	(18,759)
Portion of cost charged to expense	$ (3,077)	$ (3,078)	$ (9,230)	$ (7,348)