Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash and interest bearing deposits, restricted	$ 60.8	$ 176.1
Restricted interest-bearing deposits	90.1	102.8
Securities carried at fair value with changes recorded in net income	247.7	283.5
Other assets at fair value	71.5	111.6
Other liabilities at fair value	190.6	177.9
Borrowings contractually due within twelve months	$ 897.4	$ 2,321.7
Preferred Stock, par value (in dollars per share)	$ 0.01
Preferred Stock, shares authorized (in shares)	100,000,000
Preferred Stock, shares issued (in shares)	325,000
Preferred Stock, shares outstanding (in shares)	325,000
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (in shares)	600,000,000	600,000,000
Common Stock, shares issued (in shares)	207,439,872	206,182,213
Common Stock, shares outstanding (in shares)	131,370,803	202,087,672
Treasury stock, shares at cost (in shares)	76,069,069	4,094,541