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 and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 7,342	$ 8,345	$ 10,530
Interest Cost	13,036	15,705	15,095
Expected Return on Assets	(22,139)	(19,466)	(22,250)
Other Adjustments	0	0	0
Subtotal	(1,761)	4,584	3,375
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(247)	(281)	(351)
Amortization of Net Losses/(Gains)	6,578	8,412	6,937
Total Amortization	6,331	8,131	6,586
Net Periodic Benefit Cost	4,570	12,715	9,961
Cost of settlements	10,369	1,933	0
Total Expense for Year	14,939	14,648	9,961
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	1,263	1,345	1,160
Interest Cost	832	1,130	834
Expected Return on Assets	(7,407)	(5,785)	(6,359)
Other Adjustments	0	0	0
Subtotal	(5,312)	(3,310)	(4,365)
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	51	(34)	(4,104)
Amortization of Net Losses/(Gains)	(783)	0	(250)
Total Amortization	(732)	(34)	(4,354)
Net Periodic Benefit Cost	(6,044)	(3,344)	(8,719)
Cost of settlements	0	0	0
Total Expense for Year	$ (6,044)	$ (3,344)	$ (8,719)