Earnings Per Common Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator (basic and diluted):
Net income	$ 148	$ 134	$ 446	$ 393
Less: Allocation to unvested restricted stock awards	1	2	4	5
Net income available to common shareholders	$ 147	$ 132	$ 442	$ 388
Denominator:
Basic common shares (based upon weighted average) (in shares)	313	326	315	328
Add: Stock option dilution (in shares)	3	3	4	4
Diluted common shares (in shares)	316	329	319	332