Retirement and Post-retirement Benefit Plans - Components of the net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefit
Net Periodic Benefit Costs
Service cost	$ 2	$ 2
Interest cost	4	4
Expected return on plan assets	(6)	(6)
Recognized actuarial loss	1	2
Total	1	2
Postretirement Benefit
Net Periodic Benefit Costs
Service cost	6	5
Interest cost	9	10
Recognized actuarial loss	4	4
Total	19	19
Defined benefit plans
Net Periodic Benefit Costs
Employer contributions	$ 1	$ 1