Benefit Plans - Cash flows narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Components of net periodic cost
Defined benefit pension plans, estimated employer contributions in next fiscal year	$ 133
Employer contribution	131	$ 131
Postretirement benefits
Components of net periodic cost
Employer contribution	23	25
Participant contributions	$ 12	$ 16