Benefit Plans - Change in Accumulated Other Comprehensive Income (AOCI) and Expected Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension and Supplemental Executive Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	$ 108,808	$ 108,054
Recognized during year - Prior Service (Cost)/Credit	281	351
Recognized during year - Net Actuarial (Losses)/Gains	(8,412)	(6,937)
Occurring during year - Prior Service Cost	(5)	(15)
Occurring during year - Net Actuarial Losses/(Gains)	(150)	7,355
Occuring during year - Net Settlement Losses/(Gains)	(1,933)	0
AOCI in Current Year	98,589	108,808
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	(248)
Amortization of Net Losses/(Gains)	6,534
Other Postretirement Benefit Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI in Prior Year	3,629	(15,576)
Recognized during year - Prior Service (Cost)/Credit	34	4,104
Recognized during year - Net Actuarial (Losses)/Gains	0	250
Occurring during year - Prior Service Cost	0	3,928
Occurring during year - Net Actuarial Losses/(Gains)	(18,944)	10,923
Occuring during year - Net Settlement Losses/(Gains)	0	0
AOCI in Current Year	(15,281)	$ 3,629
Amortization of Net Transition Obligation/(Asset)	0
Amortization of Prior Service Cost/(Credit)	51
Amortization of Net Losses/(Gains)	$ (761)