Benefit Plans - Expected Future Benefit Payments (Details) - Pension plan - Nonqualified plan $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, 2018	$ 6
Defined Benefit Plan, Expected Future Benefit Payment, 2019	6
Defined Benefit Plan, Expected Future Benefit Payment, 2020	6
Defined Benefit Plan, Expected Future Benefit Payment, 2021	6
Defined Benefit Plan, Expected Future Benefit Payment, 2022	6
Defined Benefit Plan, Expected Future Benefit Payment, 2023-2027	27
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 6