Summary of Significant Accounting Policies - Effects (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and Due from Banks	$ 6,418.5	$ 3,021.0	$ 3,159.0	$ 5,332.0	$ 6,418.5
Federal Reserve Deposits					0.0
Federal Reserve and Other Central Bank Deposits				26,674.2	23,695.5
Interest-Bearing Deposits with Banks				$ 4,800.6	6,872.2
Change in Interest-Bearing Deposits with Banks	1,906.1	284.9	1,198.0
Net Change in Federal Reserve Deposits		0.0	0.0
Net Change in Federal Reserve and Other Central Bank Deposits	(4,124.2)	558.6	(2,397.8)
Net Cash Used in Investing Activities	(10,192.1)	(6,900.1)	(8,234.6)
Effect of Foreign Currency Exchange Rates on Cash	58.7	(70.9)	86.9
Increase in Cash and Due from Banks	(1,086.5)	3,397.5	(138.0)
Cash and Due from Banks at Beginning of Year	6,418.5	3,021.0	3,159.0
Cash and Due from Banks at End of Year	5,332.0	6,418.5	3,021.0
Interest-Bearing Due from and Deposits with Banks	64.3	84.9	97.2
Federal Reserve Deposits and Other		0.0	0.0
Federal Reserve and Other Central Bank Deposits and Other	109.8	70.2	71.7
Change In Presentation Of Federal Reserve and Foreign Bank Deposits | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and Due from Banks	6,444.6	3,050.6	3,162.4		6,444.6
Federal Reserve Deposits					16,398.5
Federal Reserve and Other Central Bank Deposits					0.0
Interest-Bearing Deposits with Banks					14,143.1
Change in Interest-Bearing Deposits with Banks		(144.9)	3,297.2
Net Change in Federal Reserve Deposits		987.8	(4,474.8)
Net Change in Federal Reserve and Other Central Bank Deposits		0.0	0.0
Net Cash Used in Investing Activities		(6,900.7)	(8,212.4)
Effect of Foreign Currency Exchange Rates on Cash		(73.8)	90.9
Increase in Cash and Due from Banks		3,394.0	(111.8)
Cash and Due from Banks at Beginning of Year	6,444.6	3,050.6	3,162.4
Cash and Due from Banks at End of Year		6,444.6	3,050.6
Interest-Bearing Due from and Deposits with Banks		108.3	127.6
Federal Reserve Deposits and Other		46.8	41.3
Federal Reserve and Other Central Bank Deposits and Other		0.0	0.0
Change In Presentation Of Federal Reserve and Foreign Bank Deposits | Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and Due from Banks	(26.1)	(29.6)	(3.4)		(26.1)
Federal Reserve Deposits					(16,398.5)
Federal Reserve and Other Central Bank Deposits					23,695.5
Interest-Bearing Deposits with Banks					$ (7,270.9)
Change in Interest-Bearing Deposits with Banks		429.8	(2,099.2)
Net Change in Federal Reserve Deposits		987.8	(4,474.8)
Net Change in Federal Reserve and Other Central Bank Deposits		(558.6)	2,397.8
Net Cash Used in Investing Activities		0.6	(22.2)
Effect of Foreign Currency Exchange Rates on Cash		2.9	(4.0)
Increase in Cash and Due from Banks		3.5	(26.2)
Cash and Due from Banks at Beginning of Year	$ (26.1)	(29.6)	(3.4)
Cash and Due from Banks at End of Year		(26.1)	(29.6)
Interest-Bearing Due from and Deposits with Banks		(23.4)	(30.4)
Federal Reserve Deposits and Other		(46.8)	(41.3)
Federal Reserve and Other Central Bank Deposits and Other		$ 70.2	$ 71.7