Employee Benefit Plans - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 281	$ 301
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 0	$ 0