Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 10,530	$ 9,556	$ 9,130
Interest Cost	15,095	15,475	15,906
Expected Return on Assets	(22,250)	(20,099)	(19,508)
Other Adjustments	0	0	0
Subtotal	3,375	4,932	5,528
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(351)	(426)	(687)
Amortization of Net Losses/(Gains)	6,937	6,169	5,856
Total Amortization	6,586	5,743	5,169
Net Periodic Benefit Cost	9,961	10,675	10,697
Cost of settlements	0	0	1,277
Total Expense for Year	9,961	10,675	11,974
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	1,160	813	751
Interest Cost	834	706	704
Expected Return on Assets	(6,359)	(5,248)	(4,886)
Other Adjustments	0	0	0
Subtotal	(4,365)	(3,729)	(3,431)
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(4,104)	(6,649)	(6,649)
Amortization of Net Losses/(Gains)	(250)	0	0
Total Amortization	(4,354)	(6,649)	(6,649)
Net Periodic Benefit Cost	(8,719)	(10,378)	(10,080)
Cost of settlements	0	0	0
Total Expense for Year	$ (8,719)	$ (10,378)	$ (10,080)