Earnings Per Common Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Basic and diluted:
Net income	$ 1,142.0		$ 1,266.0		$ 4,583.0		$ 4,264.0
Preferred dividends	(21.0)		(22.0)		(61.0)		(42.0)
Net income available to common stockholders - basic	1,121.0		1,244.0		4,522.0		4,222.0
Net income available to common stockholders - diluted	1,121.0		1,244.0		4,522.0		4,222.0
Earnings allocated to participating share awards	(9.0)		(10.0)		(37.0)		(32.0)
Net income attributable to common shareholders	$ 1,112.0		$ 1,234.0		$ 4,485.0		$ 4,190.0
Denominator:
Basic	920.0		994.0		940.0		1,007.0
Add: Weighted-average stock options	3.0		3.0		3.0		4.0
Diluted	923.0		997.0		943.0		1,011.0
Earnings Per Common Share, Basic (Note 15) [Abstract]
Net income attributable to common shareholders	$ 1.21	[1]	$ 1.24	[1]	$ 4.77	[2]	$ 4.16	[2]
Earnings Per Common Share, Diluted (Note 15) [Abstract]
Net income attributable to common shareholders	$ 1.2	[3]	$ 1.24	[3]	$ 4.76	[4]	$ 4.15	[4]
Earnings Per Common Share (Textuals) [Abstract]
Subordinated debentures	$ 750.0				$ 750.0
Subordinated debt issuance year	2006
Subordinated debt maturity year	2036
Stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per Share, amount	3.2		0.6		2.2		0.5

[1] Represents net income less (i) earnings allocated to participating share awards of $ 9 million and $ 10 million for the three months ended September 30 , 2016 and 2015 , respectively, and (ii) dividends on preferred shares of $ 21 million and $ 22 million for the three months ended September 30 , 2016 and 2015 , respectively.

[2] Represents net income less (i) earnings allocate d to participating share awards of $ 37 million and $ 32 million for the nine months ended September 30 , 2016 and 2015 , respectively, and (ii) dividends on preferred shares of $ 61 million and $ 42 million for the nine months ended September 30 , 2016 and 2015 , respectively.

[3] Represents net income less (i) earnings allocated to participating share awards of $ 9 million and $ 10 million for the three months ended September 30 , 2016 and 2015 , respectively, and (ii) dividends on preferred shares of $ 21 million and $ 22 million for the three months ended September 30 , 2016 and 2015 , respectively.

[4] Represents net income less (i) earnings allocate d to participating share awards of $ 37 million and $ 32 million for the nine months ended September 30 , 2016 and 2015 , respectively, and (ii) dividends on preferred shares of $ 61 million and $ 42 million for the nine months ended September 30 , 2016 and 2015 , respectively.