Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan And SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 22,392	$ 19,755	$ 21,614
Interest cost	27,853	28,417	26,096
Expected return on plan assets	(34,886)	(31,763)	(33,018)
Amortization of prior service cost	257	257	257
Net loss recognition	6,717	10,216	7,879
Net periodic benefit cost	22,333	26,882	22,828
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,902	3,006	3,188
Interest cost	6,042	5,598	4,831
Expected return on plan assets	(2,377)	(2,101)	(1,973)
Amortization of prior service cost	(958)	(981)	(1,089)
Net loss recognition	4,871	4,013	4,232
Net periodic benefit cost	$ 11,480	$ 9,535	$ 9,189