Income Per Common Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator (basic and diluted):
Net income	$ 211	$ 163	$ 360	$ 301
Less: Allocation to unvested restricted stock awards	1	2	3	3
Net income available to common shareholders	$ 210	$ 161	$ 357	$ 298
Denominator:
Basic common shares (based upon weighted average) (in shares)	306	315	308	316
Add: Stock option dilution (in shares)	3	4	3	4
Diluted common shares (in shares)	309	319	311	320