Earnings Per Common Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Basic and diluted:
Net income	$ 1,340.0		$ 2,015.0		$ 2,577.0		$ 3,441.0
Preferred dividends	(19.0)		(19.0)		(40.0)		(40.0)
Net income available to common stockholders - basic	1,321.0		1,996.0		2,537.0		3,401.0
Net income available to common stockholders - diluted	1,321.0		1,996.0		2,537.0		3,401.0
Earnings allocated to participating share awards	(11.0)		(17.0)		(21.0)		(28.0)
Net income attributable to common shareholders	$ 1,310.0		$ 1,979.0		$ 2,516.0		$ 3,373.0
Denominator:
Basic	890.0		938.0		895.0		949.0
Add: Weighted-average stock options	3.0		3.0		3.0		3.0
Diluted	893.0		941.0		898.0		952.0
Earnings Per Common Share, Basic (Note 15) [Abstract]
Net income attributable to common shareholders	$ 1.47	[1]	$ 2.11	[1]	$ 2.81	[2]	$ 3.55	[2]
Earnings Per Common Share, Diluted (Note 15) [Abstract]
Net income attributable to common shareholders	$ 1.47	[1]	$ 2.1	[1]	$ 2.8	[2]	$ 3.54	[2]
Stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per Share, amount	1.2		2.5		1.2		1.7

[1] Represents net income less (i) earnings allocated to participating share awards of $ 11 million and $ 17 million for the three months ended June 30 , 2017 and 2016 , respectively, and (ii) dividends on preferred shares of $ 19 million for both the three months ended June 30 , 2017 and 2016 .

[2] Represents net income less (i) earnings allocated to participating share awards of $ 21 million and $ 28 million for the six months ended June 30 , 2017 and 2016 , respectively, and (ii) dividends on preferred shares of $ 40 million for both the six months ended June 30 , 2017 and 2016 .