Benefit Plans - Cash flows narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pension benefits
Components of net periodic cost
Defined benefit pension plans, estimated employer contributions in next fiscal year	$ 26
Employer contribution	16	$ 131
Postretirement benefits
Components of net periodic cost
Defined benefit pension plans, estimated employer contributions in next fiscal year	0
Employer contribution	22	23
Participant contributions	$ 13	$ 12