Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	$ 10,256	$ 8,565	$ 11,338
Interest Cost	15,847	15,987	15,289
Expected Return on Assets	(21,109)	(21,030)	(20,144)
Other Adjustments	0	0	0
Subtotal	4,994	3,522	6,483
Amortization of Net Transition Obligation/(Asset)	0	0	0
Amortization of Net Prior Service Cost/(Credit)	(845)	(930)	503
Amortization of Net Losses/(Gains)	5,485	1,083	6,145
Total Amortization	4,640	153	6,648
Net Periodic Benefit Cost	9,634	3,675	13,131
Cost of settlements or curtailments	3,172	302	0
Total Expense for Year	12,806	3,977	13,131
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Service Cost	833	659	812
Interest Cost	697	653	618
Expected Return on Assets	(4,991)	(4,648)	(3,679)
Other Adjustments	0	0	0
Subtotal	(3,461)	(3,336)	(2,249)
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)	(175)	(435)	0
Total Amortization	(6,824)	(7,084)	(6,649)
Net Periodic Benefit Cost	(10,285)	(10,420)	(8,898)
Cost of settlements or curtailments	0	0	0
Total Expense for Year	$ (10,285)	$ (10,420)	$ (8,898)