Employee Benefit Plans - Weighted-average Asset Allocation by Asset Category (Details)	Dec. 31, 2016	Dec. 31, 2015
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	51.30%	59.50%
Equity
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	38.00%	32.00%
Cash and Accrued Income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	10.70%	8.50%