Securities - Available-for-Sale and Held-to-Maturity Debt Securities Gains (Losses) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt securities available-for-sale:
Amortized Cost	$ 3,545.8	$ 3,185.6
Gross Unrealized Gains	25.4	4.7
Gross Unrealized Losses	(6.9)	(69.3)
Fair Value	3,564.3	3,121.0
Debt securities held-to-maturity:
Amortized Cost	3,869.2	3,792.3
Gross Unrealized Gains	151.5	35.9
Gross Unrealized Losses	(0.7)	(52.3)
Fair Value	4,020.0	3,775.9
U.S. Treasury and Agency [Member]
Debt securities available-for-sale:
Amortized Cost	689.5	699.0
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	(2.8)	(21.1)
Fair Value	687.1	678.0
Debt securities held-to-maturity:
Amortized Cost	0.0
Fair Value	0.0
State and Municipal [Member]
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	2,503.9	2,352.4
Gross Unrealized Gains	142.0	35.4
Gross Unrealized Losses	(0.4)	(18.4)
Fair Value	2,645.5	2,369.4
GSE Mortgage-Backed Securities [Member]
Debt securities available-for-sale:
Amortized Cost	2,856.3	2,486.6
Gross Unrealized Gains	25.0	4.6
Gross Unrealized Losses	(4.1)	(48.2)
Fair Value	2,877.2	2,443.0
Debt securities held-to-maturity:
Amortized Cost	1,271.4	1,367.5
Gross Unrealized Gains	8.0	0.0
Gross Unrealized Losses	(0.3)	(33.2)
Fair Value	1,279.1	1,334.3
Corporate [Member]
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	92.4	70.9
Gross Unrealized Gains	1.5	0.5
Gross Unrealized Losses	0.0	(0.7)
Fair Value	93.9	70.7
Other [Member]
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	1.5	1.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 1.5	$ 1.5