Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic Earnings per Share
Net Income	$ 131	$ 338	$ 152	$ 247	$ 80	$ 265	$ 109	$ 273	$ 868	$ 727	$ 905
Less allocation of earnings to net restricted stock awards									(2)	(2)	(1)
Net income available to common shareholders — basic									$ 866	$ 725	$ 904
Average number of common shares outstanding, basic (in shares)									179	179	177
Basic Earnings per Share (in dollars per share)	$ 0.73	$ 1.88	$ 0.84	$ 1.38	$ 0.45	$ 1.47	$ 0.61	$ 1.53	$ 4.84	$ 4.05	$ 5.11
Diluted Earnings per Share
Net Income	$ 131	$ 338	$ 152	$ 247	$ 80	$ 265	$ 109	$ 273	$ 868	$ 727	$ 905
Less allocation of earnings to net restricted stock awards									(2)	(2)	(1)
Net income available to common shareholders — diluted									$ 866	$ 725	$ 904
Average number of common shares outstanding, diluted (in shares)									179	179	177
Diluted Earnings per Share (in dollars per share)	$ 0.73	$ 1.88	$ 0.84	$ 1.37	$ 0.45	$ 1.47	$ 0.61	$ 1.53	$ 4.83	$ 4.05	$ 5.10