Postretirement Benefits Expected Future Benefit Payments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Compensation and Retirement Disclosure [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 143
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	143
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	165
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	163
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	170
Defined Benefit Plan, Expected Future Benefit Payments, Rolling Year Five	188
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	$ 1,126