Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension benefits
Components of net periodic cost
Service cost	$ 28	$ 28	$ 85	$ 84
Interest cost	66	71	198	214
Expected return on plan assets	(102)	(99)	(306)	(298)
Amortization of prior service credit	(14)	(14)	(42)	(42)
Amortization of net actuarial loss (gain)	48	44	142	131
Settlement loss	94	7	110	23
Net periodic postretirement credit	120	37	187	112
Postretirement benefits
Components of net periodic cost
Service cost	2	2	6	7
Interest cost	4	4	11	13
Amortization of prior service credit	(6)	(5)	(18)	(16)
Amortization of net actuarial loss (gain)	(6)	(2)	(18)	(18)
Net periodic postretirement credit	$ (6)	$ (1)	$ (19)	$ (14)