Benefit Plans - Change in Accumulated Other Comprehensive Income (AOCI) and Expected Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension and Supplemental Executive Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	$ 101,575	$ 92,647
Recognized during year - Prior Service (Cost)/Credit	426	687
Recognized during year - Net Actuarial (Losses)/Gains	(6,169)	(5,856)
Occurring during year - Prior Service Cost	10	16
Occurring during year - Net Actuarial Losses/(Gains)	12,212	15,358
Occurring during year - Net Settlement Losses/(Gains)	0	(1,277)
Other adjustments	0	0
AOCI in Current Year	108,054	101,575
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	(349)
Amortization of Net Losses/(Gains)	7,140
Other Postretirement Benefit Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	(18,079)	(23,951)
Recognized during year - Prior Service (Cost)/Credit	6,649	6,649
Recognized during year - Net Actuarial (Losses)/Gains	0	0
Occurring during year - Prior Service Cost	0	0
Occurring during year - Net Actuarial Losses/(Gains)	(4,146)	(777)
Occurring during year - Net Settlement Losses/(Gains)	0	0
Other adjustments	0	0
AOCI in Current Year	(15,576)	$ (18,079)
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	(4,104)
Amortization of Net Losses/(Gains)	$ (183)