Basis of Presentation ( Effect of the Change in Presentation on Consolidated Statements of Cash Flows and Related Footnotes) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in Interest-Bearing Deposits with Banks	$ (327.8)	$ 721.9
Net Change in Federal Reserve Deposits		0.0
Net Change in Federal Reserve and Other Central Bank Deposits	767.1	1,987.3
Net Cash Provided by Investing Activities	1,026.5	662.5
Effect of Foreign Currency Exchange Rates on Cash	213.6	203.1
Increase in Cash and Due from Banks	(682.8)	569.9
Cash and Due from Banks at Beginning of Year	5,332.0	6,418.5
Cash and Due from Banks at End of Period	4,649.2	6,988.4
Interest-Bearing Deposits with Banks	5,247.8	17.5	$ 4,800.6
Federal Reserve Bank Stock		0.0
Federal Reserve and Other Central Bank Deposits	$ 25,959.5	29.2	$ 26,674.2
Change In Presentation Of Federal Reserve and Foreign Bank Deposits | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in Interest-Bearing Deposits with Banks		716.7
Net Change in Federal Reserve Deposits		1,983.2
Net Change in Federal Reserve and Other Central Bank Deposits		0.0
Net Cash Provided by Investing Activities		653.2
Effect of Foreign Currency Exchange Rates on Cash		202.9
Increase in Cash and Due from Banks		560.4
Cash and Due from Banks at Beginning of Year		6,444.6
Cash and Due from Banks at End of Period		7,005.0
Interest-Bearing Deposits with Banks		23.7
Federal Reserve Bank Stock		23.0
Federal Reserve and Other Central Bank Deposits		0.0
Change In Presentation Of Federal Reserve and Foreign Bank Deposits | Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in Interest-Bearing Deposits with Banks		5.2
Net Change in Federal Reserve Deposits		(1,983.2)
Net Change in Federal Reserve and Other Central Bank Deposits		1,987.3
Net Cash Provided by Investing Activities		9.3
Effect of Foreign Currency Exchange Rates on Cash		0.2
Increase in Cash and Due from Banks		9.5
Cash and Due from Banks at Beginning of Year		(26.1)
Cash and Due from Banks at End of Period		(16.6)
Interest-Bearing Deposits with Banks		(6.2)
Federal Reserve Bank Stock		(23.0)
Federal Reserve and Other Central Bank Deposits		$ 29.2