Securities - Continuous Unrealized Loss Position on Available-for-Sale and Held-to-Maturities Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale, Fair Value
Less Than 12 Months	$ 746.4	$ 132.4
12 Months Or Longer	871.7	2,312.5
Total	1,618.1	2,444.9
Available-for-sale, Unrealized Losses
Less Than 12 Months	(3.5)	(0.5)
12 Months Or Longer	(3.4)	(68.8)
Total	(6.9)	(69.3)
Held-to-maturity, Fair Value
Less Than 12 Months	137.4	144.6
12 Months Or Longer	29.1	2,034.6
Total	166.5	2,179.2
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(0.5)	(1.3)
12 Months Or Longer	(0.2)	(51.0)
Total	(0.7)	(52.3)
GSE Mortgage-Backed Securities [Member]
Available-for-sale, Fair Value
Less Than 12 Months	565.4	132.4
12 Months Or Longer	509.3	1,656.3
Total	1,074.7	1,788.7
Available-for-sale, Unrealized Losses
Less Than 12 Months	(3.1)	(0.5)
12 Months Or Longer	(1.0)	(47.7)
Total	(4.1)	(48.2)
Held-to-maturity, Fair Value
Less Than 12 Months	100.9	0.0
12 Months Or Longer	9.1	1,334.3
Total	110.0	1,334.3
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(0.3)	0.0
12 Months Or Longer	0.0	(33.2)
Total	(0.3)	(33.2)
U.S. Treasury and Agency [Member]
Available-for-sale, Fair Value
Less Than 12 Months	181.0	0.0
12 Months Or Longer	362.4	656.2
Total	543.4	656.2
Available-for-sale, Unrealized Losses
Less Than 12 Months	(0.4)	0.0
12 Months Or Longer	(2.4)	(21.1)
Total	(2.8)	(21.1)
State and Municipal [Member]
Held-to-maturity, Fair Value
Less Than 12 Months	33.0	113.4
12 Months Or Longer	11.4	697.6
Total	44.4	811.0
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(0.2)	(0.7)
12 Months Or Longer	(0.2)	(17.7)
Total	(0.4)	(18.4)
Corporate [Member]
Held-to-maturity, Fair Value
Less Than 12 Months	3.5	31.2
12 Months Or Longer	8.6	2.7
Total	12.1	33.9
Held-to-maturity, Unrealized Losses
Less Than 12 Months	0.0	(0.6)
12 Months Or Longer	0.0	(0.1)
Total	0.0	$ (0.7)
Maximum [Member]
Available-for-sale, Unrealized Losses
Total	$ (0.1)