Pension and Other Postretirement Benefits - Summary of Accumulated Other Comprehensive Loss That Have Not Been Recognized as Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 44,359	$ 49,260
Net prior service cost	11	(44)
Total	44,370
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	(1,545)	1,842
Net prior service cost	0	$ 0
Total	$ (1,545)