Employee Benefit Plans - Weighted-average Asset Allocation by Asset Category (Details)	Dec. 31, 2018	Dec. 31, 2017
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	68.00%	61.70%
Equity
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	25.20%	29.50%
Cash and Accrued Income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual asset allocations	6.80%	8.80%