Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 29	$ 84	$ 86
Interest cost	71	64	214	192
Expected return on plan assets	(99)	(106)	(298)	(318)
Amortization of prior service credit	(14)	(14)	(42)	(42)
Amortization of net actuarial loss (gain)	44	47	131	143
Settlement loss	7	6	23	18
Net periodic postretirement (credit) cost	37	26	112	79
Postretirement benefits
Components of net periodic cost
Service cost	2	3	7	9
Interest cost	4	6	13	17
Amortization of prior service credit	(5)	(5)	(16)	(16)
Amortization of net actuarial loss (gain)	(2)	(3)	(18)	(7)
Net periodic postretirement (credit) cost	$ (1)	$ 1	$ (14)	$ 3