Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,550	$ 4,948	$ 11,096	$ 9,822
Interest cost	6,967	7,100	13,938	14,238
Expected return on plan assets	(8,625)	(7,953)	(17,750)	(15,768)
Amortization of prior service cost	75	75	150	150
Net loss recognition	1,679	2,426	3,333	4,841
Net periodic benefit cost	5,646	6,596	10,767	13,283
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	980	753	1,959	1,525
Interest cost	1,524	1,164	3,039	2,536
Expected return on plan assets	(590)	(611)	(1,220)	(1,329)
Amortization of prior service cost	(275)	(275)	(550)	(550)
Net loss recognition	1,243	1,329	2,486	2,575
Net periodic benefit cost	$ 2,882	$ 2,360	$ 5,714	$ 4,757