Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension
Components of net periodic benefit cost:
Service cost	$ 16	$ 18
Interest cost	23	23
Expected return on plan assets	(37)	(38)
Amortization of actuarial (gain) loss	11	15
Net periodic benefit (income) cost	13	18
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	3
Interest cost	6	6
Expected return on plan assets	(6)	(6)
Amortization of actuarial (gain) loss	0	0
Net periodic benefit (income) cost	$ 2	$ 3