Pension and Other Postretirement Benefits - Expected Amortization of Accumulated Other Comprehensive Loss as Components of Net Periodic Benefit Cost (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	$ (41)
Amortization of actuarial loss	2,758
Total	2,717
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	0
Amortization of actuarial loss	0
Total	$ 0