Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 9,556	$ 9,130	$ 10,256
Interest Cost	15,475	15,906	15,847
Expected Return on Assets	(20,099)	(19,508)	(21,109)
Other Adjustments	0	0	0
Subtotal	4,932	5,528	4,994
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(426)	(687)	(845)
Amortization of Net Losses/(Gains)	6,169	5,856	5,485
Total Amortization	5,743	5,169	4,640
Net Periodic Benefit Cost	10,675	10,697	9,634
Cost of settlements	0	1,277	3,172
Total Expense for Year	10,675	11,974	12,806
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	813	751	833
Interest Cost	706	704	697
Expected Return on Assets	(5,248)	(4,886)	(4,991)
Other Adjustments	0	0	0
Subtotal	(3,729)	(3,431)	(3,461)
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(6,649)	(6,649)	(6,649)
Amortization of Net Losses/(Gains)	0	0	(175)
Total Amortization	(6,649)	(6,649)	(6,824)
Net Periodic Benefit Cost	(10,378)	(10,080)	(10,285)
Cost of settlements	0	0	0
Total Expense for Year	$ (10,378)	$ (10,080)	$ (10,285)