Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 29
Interest cost	71	64
Expected return on plan assets	(99)	(106)
Amortization of prior service credit	(14)	(14)
Amortization of net actuarial loss (gain)	43	48
Settlement loss	8	6
Net periodic postretirement (credit) cost	37	27
Postretirement benefits
Components of net periodic cost
Service cost	2	3
Interest cost	4	6
Amortization of prior service credit	(5)	(6)
Amortization of net actuarial loss (gain)	(8)	(2)
Net periodic postretirement (credit) cost	$ (7)	$ 1