Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension benefits
Components of net periodic cost
Service cost	$ 29	$ 24	$ 86	$ 72
Interest cost	64	63	192	191
Expected return on plan assets	(106)	(99)	(318)	(298)
Amortization of prior service credit	(14)	(14)	(42)	(43)
Amortization of net actuarial loss (gain)	47	29	143	86
Settlement loss	6	11	18	33
Net periodic postretirement cost (credit)	26	14	79	41
Postretirement benefits
Components of net periodic cost
Service cost	3	3	9	8
Interest cost	6	6	17	17
Amortization of prior service credit	(5)	(6)	(16)	(17)
Amortization of net actuarial loss (gain)	(3)	(6)	(7)	(17)
Net periodic postretirement cost (credit)	$ 1	$ (3)	$ 3	$ (9)