Retirement and Postretirement Benefit Plans - Components of the net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefit
Net Periodic Benefit Costs
Service cost	$ 2	$ 2	$ 4	$ 4
Interest cost	4	4	8	8
Expected return on plan assets	(6)	(6)	(12)	(12)
Recognized actuarial loss	1	2	2	4
Total	1	2	2	4
Postretirement Benefit
Net Periodic Benefit Costs
Service cost	6	5	12	10
Interest cost	9	10	18	20
Recognized actuarial loss	6	4	10	8
Total	21	19	40	38
Defined benefit plans
Net Periodic Benefit Costs
Employer contributions	$ 1	$ 1	$ 2	$ 2