Earnings Per Common Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator (basic and diluted):
Net income	$ 158	$ 150	$ 298	$ 259
Less: Allocation to unvested restricted stock awards	1	2	3	3
Net income available to common shareholders	$ 157	$ 148	$ 295	$ 256
Denominator:
Basic common shares (based upon weighted average) (in shares)	315	328	316	329
Add: Stock option dilution (in shares)	4	3	4	4
Diluted common shares (in shares)	319	331	320	333