Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 13,715	$ 13,448	$ 41,144	$ 40,344
Interest cost on benefit obligation	32,439	32,912	97,316	98,735
Expected return on plan assets	(43,568)	(43,477)	(130,703)	(130,429)
Amortization of:
Prior service cost (credit)	20	132	61	395
Net actuarial loss	11,975	10,179	35,924	30,538
Net periodic benefit cost	14,581	13,194	43,742	39,583
Portion of cost charged to expense	7,231	6,476	21,692	19,427
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,280	3,748	12,839	11,245
Interest cost on benefit obligation	7,490	7,430	22,470	22,291
Expected return on plan assets	(13,350)	(9,123)	(40,051)	(27,371)
Amortization of:
Prior service cost (credit)	(9,461)	(9,471)	(28,382)	(28,413)
Net actuarial loss	1,279	1,147	3,838	3,442
Net periodic benefit cost	(9,762)	(6,269)	(29,286)	(18,806)
Portion of cost charged to expense	$ (4,841)	$ (3,077)	$ (14,523)	$ (9,230)