Employee Benefits - Schedule of Expected Cash Flow for Pension and Post Retirement Benefit Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
To plan trusts | Pension Plan Asset
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions	$ 39
To plan trusts | Postretirement Benefit Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions	0
To plan participants | Pension Plan Asset
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions	2
To plan participants | Postretirement Benefit Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions	$ 0