Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension
Components of net periodic benefit cost:
Service cost	$ 16	$ 18	$ 32	$ 36
Interest cost	24	23	47	46
Expected return on plan assets	(36)	(38)	(73)	(76)
Amortization of actuarial loss	10	15	21	30
Amortization of prior service income	0	0	0	0
Net periodic benefit cost	14	18	27	36
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	3	2	5	5
Interest cost	6	6	12	12
Expected return on plan assets	(5)	(6)	(11)	(12)
Amortization of actuarial loss	0	0	0	0
Amortization of prior service income	(1)	0	(1)	0
Net periodic benefit cost	$ 3	$ 2	$ 5	$ 5