Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 105,019
Available-for-Sale, Maturity, One to five years, Amortized Cost	518,087
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	583,312
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	3,350,187
Available-for-Sale, Amortized Cost	4,556,605	$ 4,172,617
Held-to-Maturity, Maturity, Within one year, Amortized Cost	18,069
Held-to-Maturity, Maturity, One to five years, Amortized Cost	34,127
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	76,905
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	355,733
Held-to-Maturity, Amortized Cost	484,834	506,334	$ 535,153
Available-for-Sale, Maturity, Within one year, Fair Value	105,091
Available-for-Sale, Maturity, One to five years, Fair Value	518,110
Available-for-Sale, Maturity, Five to ten years, Fair Value	590,229
Available-for-Sale, Maturity, Beyond ten years, Fair Value	3,356,342
Available-for-Sale securities, Fair Value	4,569,772	4,123,416	3,219,863
Held-to-Maturity, Maturity, Within one year, Fair Value	18,179
Held-to-Maturity, Maturity, One to five years, Fair Value	35,080
Held-to-Maturity, Maturity, Five to ten years, Fair Value	79,823
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	360,795
Held-to-Maturity, Fair Value	$ 493,877	$ 506,103	$ 536,143
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.49%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.46%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	3.28%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.97%
Available-for-Sale, Weighted Average Yield	2.94%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	4.32%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	3.95%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.49%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	3.58%
Held-to-Maturity, Weighted Average Yield	3.78%

Weighted average yield on total available-for-sale securities.

No definition available.

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Weighted average yield on total held-to-maturity securities.

+ References

No definition available.

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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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.

+ References

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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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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 after 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 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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing after 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.

+ References