Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 9,130	$ 10,256	$ 8,565
Interest Cost	15,906	15,847	15,987
Expected Return on Assets	(19,508)	(21,109)	(21,030)
Other Adjustments	0	0	0
Subtotal	5,528	4,994	3,522
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(687)	(845)	(930)
Amortization of Net Losses/(Gains)	5,856	5,485	1,083
Total Amortization	5,169	4,640	153
Net Periodic Benefit Cost	10,697	9,634	3,675
Cost of settlements or curtailments	1,277	3,172	302
Total Expense for Year	11,974	12,806	3,977
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	751	833	659
Interest Cost	704	697	653
Expected Return on Assets	(4,886)	(4,991)	(4,648)
Other Adjustments	0	0	0
Subtotal	(3,431)	(3,461)	(3,336)
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	(175)	(435)
Total Amortization	(6,649)	(6,824)	(7,084)
Net Periodic Benefit Cost	(10,080)	(10,285)	(10,420)
Cost of settlements or curtailments	0	0	0
Total Expense for Year	$ (10,080)	$ (10,285)	$ (10,420)