Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amortization of:
Portion of cost (credit) charged to expense	$ (12,449)	$ (6,534)	$ (37,314)	$ (19,601)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,167	13,715	42,501	41,144
Interest cost on benefit obligation	31,172	32,439	93,517	97,316
Expected return on plan assets	(45,713)	(43,568)	(137,140)	(130,703)
Amortization of:
Prior service cost (credit)	0	20	0	61
Net actuarial loss	8,021	11,975	24,062	35,924
Net periodic benefit cost (credit)	7,647	14,581	22,940	43,742
Portion of cost (credit) charged to expense	2,524	7,231	7,535	21,692
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,275	4,280	15,825	12,839
Interest cost on benefit obligation	7,037	7,490	21,111	22,470
Expected return on plan assets	(10,520)	(13,350)	(31,561)	(40,051)
Amortization of:
Prior service cost (credit)	(9,461)	(9,461)	(28,382)	(28,382)
Net actuarial loss	0	1,279	0	3,838
Net periodic benefit cost (credit)	(7,669)	(9,762)	(23,007)	(29,286)
Portion of cost (credit) charged to expense	$ (5,359)	$ (4,841)	$ (16,083)	$ (14,523)