Employee Benefit Plans - Weighted-average Asset Allocation by Asset Category (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	100.00%	100.00%
Fixed Income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	61.70%	51.30%
Equity
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	29.50%	38.00%
Cash and Accrued Income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	8.80%	10.70%