Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Cash and interest bearing deposits, restricted	$ 29.6	$ 875.2
Restricted interest-bearing deposits	2.1	2.2
Securities carried at fair value with changes recorded in net income	0.0	47.2
Other assets at fair value	546.5	190.7
Other liabilities at fair value	164.4	100.8
Borrowings contractually due within twelve months	$ 514.0	$ 14.3
Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Stock, shares authorized (in shares)	100,000,000	100,000,000
Preferred Stock, shares issued (in shares)	8,325,000	8,325,000
Preferred Stock, shares outstanding (in shares)	8,325,000	8,325,000
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)	163,014,943	162,188,287
Common Stock, shares outstanding (in shares)	98,366,114	94,742,564
Treasury stock, shares at cost (in shares)	64,648,829	67,445,723