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Investment Securities - Additional Information (Detail)	12 Months Ended
Investment Securities - Additional Information (Detail)	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Investment Securities [Line Items]
Equity securities	$ 41,410,000	$ 0
Number of security positions in the securities available for sale portfolio in an unrealized loss position \| security	182	545
Fair value of securities available for sale pledged as collateral	$ 1,000,000,000.0
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years	5 years 8 months 12 days
Number of security positions in the securities held to maturity portfolio in an unrealized loss position \| security	82	378
Fair value of investments held to maturity pledged as collateral	$ 1,400,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years	5 years 2 months 12 days
Amortized cost	$ 1,558,787,000	$ 1,796,404,000
Available for sale securities	1,566,801,000	1,749,544,000
Other-than-temporary impairment losses	2,900,000	$ 0	$ 0
US States And Political Subdivisions Debt Securities, Special Revenue Bonds, Impaired Securities
Investment Securities [Line Items]
Amortized cost	680,000
Available for sale securities	$ 680,000
Non-rated single issuer securities
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position \| security	4
Other-than-temporary impaired securities
Investment Securities [Line Items]
Amortized cost	$ 294,900,000
Available for sale securities	$ 299,000,000.0
Other-than-temporary impaired securities \| Obligations of states and political subdivisions
Investment Securities [Line Items]
Investment carrying value percentage	41.50%
Investment carrying value	$ 670,800,000