Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	$ 417,770	$ 369,808
Company Service Cost	10,530	9,556	$ 9,130
Interest Cost	15,095	15,475	15,906
Plan Participants' Contributions	0	0
Net Actuarial (Gain)/Loss due to Assumption Changes	(36,132)	38,496
Net Actuarial (Gain)/Loss due to Plan Experience	2,487	2,338
Benefit Payments from Fund	(32,674)	(17,578)
Benefit Payments Directly by Company	(908)	(335)
Plan Amendments	(15)	10
Other Adjustment	0	0
Benefit Obligation at End of Year	376,153	417,770	369,808
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	24,716	17,378
Company Service Cost	1,160	813	751
Interest Cost	834	706	704
Plan Participants' Contributions	475	395
Net Actuarial (Gain)/Loss due to Assumption Changes	(1,209)	5,981
Net Actuarial (Gain)/Loss due to Plan Experience	(692)	924
Benefit Payments from Fund	(1,077)	(1,404)
Benefit Payments Directly by Company	0	0
Plan Amendments	3,928	0
Other Adjustment	(50)	(77)
Benefit Obligation at End of Year	$ 28,085	$ 24,716	$ 17,378