Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,900	$ 2,600	$ 5,900	$ 5,200
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,779	2,469	5,598	4,925
Interest cost	9,221	11,563	18,523	23,155
Expected return on plan assets	(19,211)	(17,418)	(38,575)	(34,854)
Amortization of prior service cost (credit)	825	725	1,650	1,450
Net periodic benefit (income) cost	(6,386)	(2,661)	(12,804)	(5,324)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	161	144	327	289
Interest cost	404	585	814	1,170
Expected return on plan assets	(672)	(628)	(1,344)	(1,255)
Amortization of prior service cost (credit)	(49)	(77)	(97)	(155)
Net periodic benefit (income) cost	$ (156)	$ 24	$ (300)	$ 49