Consolidated Statements of Condition - (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Debt securities, allowance for credit loss	$ 1.9
Securities held to maturity, at fair value	$ 4,065.2	$ 4,020.0
Preferred stock, par value (USD 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 (USD 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)	533,500,000	532,800,000
Unallocated common stock of Employee Stock Ownership Plan (in shares)	5,836,579	5,900,000
Treasury stock (in shares)	109,000,000.0	89,200,000