Benefit Plans - Change in Accumulated Other Comprehensive Income (AOCI) and Expected Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	$ 92,647	$ 89,390
Recognized during year - Prior Service (Cost)/Credit	687	845
Recognized during year - Net Actuarial (Losses)/Gains	(5,856)	(5,485)
Occurring during year - Prior Service Cost	16	19
Occurring during year - Net Actuarial Losses/(Gains)	15,358	11,050
Occurring during year - Net Settlement Losses/(Gains)	(1,277)	(3,172)
Other adjustments	0	0
AOCI in Current Year	101,575	92,647
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	(428)
Amortization of Net Losses/(Gains)	6,141
Other Postretirement Benefit Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	(23,951)	(33,511)
Recognized during year - Prior Service (Cost)/Credit	6,649	6,649
Recognized during year - Net Actuarial (Losses)/Gains	0	175
Occurring during year - Prior Service Cost	0	0
Occurring during year - Net Actuarial Losses/(Gains)	(777)	2,736
Occurring during year - Net Settlement Losses/(Gains)	0	0
Other adjustments	0	0
AOCI in Current Year	(18,079)	$ (23,951)
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	(6,649)
Amortization of Net Losses/(Gains)	$ 0