Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash and interest bearing deposits, restricted	$ 42.9	$ 176.1
Restricted interest-bearing deposits	81.8	102.8
Securities carried at fair value with changes recorded in net income	0.4	283.5
Other assets at fair value	68.7	111.6
Other liabilities at fair value	198.1	177.9
Borrowings contractually due within twelve months	$ 1,626.3	$ 2,321.7
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)	207,628,491	206,182,213
Common stock, shares outstanding (in shares)	131,352,924	202,087,672
Treasury stock, shares at cost (in shares)	76,275,567	4,094,541