Benefit Plans - Fair Value of pension plan assets (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic cost
Total plan assets at fair value	$ 2,882	$ 2,233
Total plan assets at fair value, allocation (as a percent)	100.00%	100.00%
Investments measured using the Net Asset Value practical expedient	$ 3,598	$ 3,525
Securities lending obligation	(216)	(158)
Other net plan assets	20	50
Fair value of plan assets	6,284	5,650	$ 5,353
Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	$ 418	$ 360
Total plan assets at fair value, allocation (as a percent)	14.50%	16.10%
Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	$ 2,425	$ 1,863
Total plan assets at fair value, allocation (as a percent)	84.10%	83.40%
Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	$ 39	$ 10
Total plan assets at fair value, allocation (as a percent)	1.40%	0.50%
Fair value of plan assets	$ 39	$ 10	18	$ 27
Equity securities
Components of net periodic cost
Total plan assets at fair value	419	302
Equity securities | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	126	155
Equity securities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	264	147
Equity securities | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	29	0
Fair value of plan assets	29	0	1	1
U.S. government and agencies
Components of net periodic cost
Total plan assets at fair value	594	315
U.S. government and agencies | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	174	30
U.S. government and agencies | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	420	285
U.S. government and agencies | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Corporate
Components of net periodic cost
Total plan assets at fair value	1,553	1,319
Corporate | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	0	0
Corporate | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	1,543	1,309
Corporate | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	10	10
Fair value of plan assets	10	10	$ 10	$ 12
Short-term investments
Components of net periodic cost
Total plan assets at fair value	294	265
Short-term investments | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	97	144
Short-term investments | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	197	121
Short-term investments | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Cash and cash equivalents
Components of net periodic cost
Total plan assets at fair value	21	32
Cash and cash equivalents | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	21	32
Cash and cash equivalents | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	0	0
Cash and cash equivalents | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Free Standing Derivative Assets
Components of net periodic cost
Total plan assets at fair value	1	0
Free Standing Derivative Assets | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	0	(1)
Free Standing Derivative Assets | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	1	1
Free Standing Derivative Assets | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	$ 0
EMA limited partnerships
Components of net periodic cost
Investments measured using the Net Asset Value practical expedient	3,200
Limited partnership interests
Components of net periodic cost
Investments measured using the Net Asset Value practical expedient	$ 402