Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Amortization of:
Portion of cost (credit) charged to expense	$ (14,118)	$ (5,752)	$ (42,171)	$ (17,240)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,058	12,476	42,174	37,427
Interest cost on benefit obligation	29,642	34,211	88,925	102,632
Expected return on plan assets	(46,861)	(42,971)	(140,582)	(128,913)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss	8,653	10,646	25,959	31,938
Net periodic benefit cost (credit)	5,492	14,362	16,476	43,084
Portion of cost (credit) charged to expense	736	7,593	2,349	22,837
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,559	4,593	16,677	13,777
Interest cost on benefit obligation	6,464	7,473	19,393	22,420
Expected return on plan assets	(10,019)	(9,603)	(30,057)	(28,809)
Amortization of:
Prior service credit	(9,394)	(9,456)	(28,182)	(28,366)
Net actuarial loss	0	0	0	0
Net periodic benefit cost (credit)	(7,390)	(6,993)	(22,169)	(20,978)
Portion of cost (credit) charged to expense	$ (5,286)	$ (4,966)	$ (15,798)	$ (14,846)