Pension and Other Post-Retirement Benefit Plans (Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 4,476	$ 6,263
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	106,742	0
Estimated future employer contributions	22,400
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2017	31,066
2018	36,615
2019	36,650
2020	39,032
2021	41,113
Years 2022 to 2026	225,994
Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	412	667
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0	$ 560
Estimated future employer contributions	400
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2017	414
2018	483
2019	502
2020	530
2021	508
Years 2022 to 2026	$ 2,177