Employee Benefit Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Parenthetical) (Details) - Private Equity Securities - Significant unobservable input (Level 3) $ in Millions	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment commitment	$ 15.0
Unfunded portion of investment commitment	$ 5.0