Benefit Plans - Development of Funded Status (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	$ 411,996	$ 360,423
Projected Benefit Obligation	(417,770)	(369,808)	$ (349,483)
Plan Assets at Fair Value	401,142	360,900	350,107
Funded Status - Overfunded/(Underfunded)	(16,628)	(8,908)
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	24,716	17,378
Projected Benefit Obligation	(24,716)	(17,378)	(16,423)
Plan Assets at Fair Value	85,303	70,408	$ 65,568
Funded Status - Overfunded/(Underfunded)	$ 60,587	$ 53,030