Consolidated Statements of Condition (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Securities held to maturity, at fair value	$ 4,020.0	$ 3,775.9
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	50,000,000.0	50,000,000.0
Preferred stock, shares issued (in shares)	10,000,000.0	10,000,000.0
Preferred stock, shares outstanding (in shares)	10,000,000.0	10,000,000.0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,950,000,000	1,950,000,000
Common stock, shares issued (in shares)	532,800,000	466,300,000
Unallocated common stock of Employee Stock Ownership Plan (in shares)	5,923,692	6,300,000
Treasury stock (in shares)	89,200,000	89,000,000.0