Investment Securities Available for Sale - Fair Value and Amortized Cost of Investment Securities by Contractual Maturity (Detail) $ in Thousands	Mar. 31, 2020 USD ($)
Investments Debt And Equity Securities [Abstract]
Investment securities available for sale, Due in one year or less, Fair Value	$ 102,204
Investment securities available for sale, Due after one year through five years, Fair Value	131,668
Investment securities available for sale, Due after five years through ten years, Fair Value	45,763
Investment securities available for sale, Due after ten years through thirty years, Fair Value	70,961
Investment securities available for sale, Mortgage backed securities, Fair Value	1,787,846
Fair value	2,138,442
Investment securities available for sale, Due one year or less, Amortized Cost	102,191
Investment securities available for sale, Due after one year through five years, Amortized Cost	131,295
Investment securities available for sale, Due after five years through ten years, Amortized Cost	44,017
Investment securities available for sale, Due after ten years through thirty years, Amortized Cost	65,503
Investment securities available for sale, Mortgage backed securities, Amortized Cost	1,711,738
Amortized Cost	$ 2,054,744

Mortgage backed securities at amortized cost.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth fiscal year through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amortized cost of investment in debt security measured at fair value with change recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

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