Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,500	$ 2,100	$ 8,500	$ 7,300
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,382	1,914	7,980	6,839
Interest cost	9,359	11,567	27,882	34,722
Expected return on plan assets	(19,368)	(17,490)	(57,943)	(52,344)
Amortization of prior service cost (credit)	877	727	2,527	2,177
Net periodic benefit (income) cost	(6,750)	(3,282)	(19,554)	(8,606)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	153	145	480	434
Interest cost	393	586	1,207	1,756
Expected return on plan assets	(675)	(627)	(2,019)	(1,882)
Amortization of prior service cost (credit)	(48)	(78)	(145)	(233)
Net periodic benefit (income) cost	$ (177)	$ 26	$ (477)	$ 75