Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 78,428
Available-for-Sale, Maturity, One to five years, Amortized Cost	484,883
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	444,926
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,360,288
Available-for-Sale, Amortized Cost	3,368,525	$ 3,252,586
Held-to-Maturity, Maturity, Within one year, Amortized Cost	26,046
Held-to-Maturity, Maturity, One to five years, Amortized Cost	32,394
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	79,600
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	374,167
Held-to-Maturity, Amortized Cost	512,207	684,063	$ 688,951
Available-for-Sale, Maturity, Within one year, Fair Value	78,457
Available-for-Sale, Maturity, One to five years, Fair Value	476,762
Available-for-Sale, Maturity, Five to ten years, Fair Value	439,284
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,277,953
Available-for-Sale securities, Fair Value	3,272,456	3,196,207	2,775,837
Held-to-Maturity, Maturity, Within one year, Fair Value	26,176
Held-to-Maturity, Maturity, One to five years, Fair Value	32,869
Held-to-Maturity, Maturity, Five to ten years, Fair Value	81,252
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	365,269
Held-to-Maturity, Fair Value	$ 505,566	$ 727,703	$ 740,564
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.46%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.29%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	3.15%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.65%
Available-for-Sale, Weighted Average Yield	2.66%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	3.70%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.07%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.39%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	3.58%
Held-to-Maturity, Weighted Average Yield	3.74%

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

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-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.

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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