Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Amortization of:
Portion of cost (credit) charged to expense	$ (5,752)	$ (12,449)	$ (17,240)	$ (37,314)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	12,476	14,167	37,427	42,501
Interest cost on benefit obligation	34,211	31,172	102,632	93,517
Expected return on plan assets	(42,971)	(45,713)	(128,913)	(137,140)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss	10,646	8,021	31,938	24,062
Net periodic benefit cost (credit)	14,362	7,647	43,084	22,940
Portion of cost (credit) charged to expense	7,593	2,524	22,837	7,535
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,593	5,275	13,777	15,825
Interest cost on benefit obligation	7,473	7,037	22,420	21,111
Expected return on plan assets	(9,603)	(10,520)	(28,809)	(31,561)
Amortization of:
Prior service credit	(9,456)	(9,461)	(28,366)	(28,382)
Net actuarial loss	0	0	0	0
Net periodic benefit cost (credit)	(6,993)	(7,669)	(20,978)	(23,007)
Portion of cost (credit) charged to expense	$ (4,966)	$ (5,359)	$ (14,846)	$ (16,083)