Investment Securities - Amortized Cost and Fair Value of Investment Securities by Contractual Maturities (Detail) - USD ($) $ in Millions	Mar. 31, 2022		Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 41.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	18.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	882.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	41.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	18.0
Held-to-maturity Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value	7.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	7.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	880.5
Debt Securities, Held-to-maturity, Maturity, without Single Maturity Date, Fair Value	192.2
Debt Securities, Held-to-maturity, Maturity, without Single Maturity Date, Amortized Cost	211.1
Debt Securities, Held-to-maturity	1,161.0		$ 1,107.1
Debt Securities, Held-to-maturity, Fair Value	1,139.9		1,146.4
Securities available for sale, Due in one year or less, Amortized Cost	1.5
Securities available for sale, After one year through five years, Amortized Cost	120.6
Securities available for sale, After five years through ten years, Amortized Cost	1,055.7
Securities available for sale, After ten years, Amortized Cost	2,405.0
Securities available for sale, Mortgage backed securities, Amortized Cost	3,582.7
Securities available for sale Total, Amortized Cost	7,165.5		6,166.5
Securities available for sale, Due in one year or less, Estimated Fair Value	1.5
Securities available for sale, After one year through five years, Estimated Fair Value	117.9
Securities available for sale, After five years through ten years, Estimated Fair Value	1,046.1
Securities available for sale, After ten years, Estimated Fair Value	2,376.3
Securities available for sale, Mortgage backed securities, Estimated Fair Value	3,310.4
Securities available for sale Total, Estimated Fair Value	$ 6,852.2	[1]	$ 6,188.8	[2]

[1] ratings differ, the Company uses an average of the available ratings by major credit agencies.

[2] For rated securities, if ratings differ, the Company uses an average of the available ratings by major credit agencies.