Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	$ 369,808	$ 349,483
Company Service Cost	9,556	9,130	$ 10,256
Interest Cost	15,475	15,906	15,847
Plan Participants' Contributions	0	0
Net Actuarial (Gain)/Loss due to Assumption Changes	38,496	14,450
Net Actuarial (Gain)/Loss due to Plan Experience	2,338	5,428
Benefit Payments from Fund	(17,578)	(21,831)
Benefit Payments Directly by Company	(335)	(2,669)
Plan Amendments	10	16
Other Adjustment	0	(105)
Benefit Obligation at End of Year	417,770	369,808	349,483
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	17,378	16,423
Company Service Cost	813	751	833
Interest Cost	706	704	697
Plan Participants' Contributions	395	408
Net Actuarial (Gain)/Loss due to Assumption Changes	5,981	497
Net Actuarial (Gain)/Loss due to Plan Experience	924	357
Benefit Payments from Fund	(1,404)	(1,678)
Benefit Payments Directly by Company	0	0
Plan Amendments	0	0
Other Adjustment	(77)	(84)
Benefit Obligation at End of Year	$ 24,716	$ 17,378	$ 16,423