Pension and Other Post-Retirement Benefit Plans (Fair Value Measurements Using Significant Level 3 Unobservable Inputs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Hedge Funds & Private Equity [Member] | Measured at net asset value [Domain]
Schedule of Level Three Defined Benefit Plan Assets [Roll Forward]
Plan assets, beginning balance	$ 25,831
Plan assets, ending balance	27,197	$ 25,831
Real Estate Investments [Member] | Measured at net asset value [Domain]
Schedule of Level Three Defined Benefit Plan Assets [Roll Forward]
Purchases & settlements	0	33,442
Pension Plan [Member]
Schedule of Level Three Defined Benefit Plan Assets [Roll Forward]
Plan assets, beginning balance	715,885	672,153
Unrealized gains	3,547
Plan assets, ending balance	681,573	715,885
Pension Plan [Member] | Measured at net asset value [Domain]
Schedule of Level Three Defined Benefit Plan Assets [Roll Forward]
Plan assets, beginning balance	59,273
Plan assets, ending balance	$ 27,197	$ 59,273