Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 8,345	$ 10,530	$ 9,556
Interest Cost	15,705	15,095	15,475
Expected Return on Assets	(19,466)	(22,250)	(20,099)
Other Adjustments	0	0	0
Subtotal	4,584	3,375	4,932
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(281)	(351)	(426)
Amortization of Net Losses/(Gains)	8,412	6,937	6,169
Total Amortization	8,131	6,586	5,743
Net Periodic Benefit Cost	12,715	9,961	10,675
Cost of settlements	1,933	0	0
Total Expense for Year	14,648	9,961	10,675
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	1,345	1,160	813
Interest Cost	1,130	834	706
Expected Return on Assets	(5,785)	(6,359)	(5,248)
Other Adjustments	0	0	0
Subtotal	(3,310)	(4,365)	(3,729)
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(34)	(4,104)	(6,649)
Amortization of Net Losses/(Gains)	0	(250)	0
Total Amortization	(34)	(4,354)	(6,649)
Net Periodic Benefit Cost	(3,344)	(8,719)	(10,378)
Cost of settlements	0	0	0
Total Expense for Year	$ (3,344)	$ (8,719)	$ (10,378)