Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 60,410
Available-for-Sale, Maturity, One to five years, Amortized Cost	511,930
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	383,847
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,303,253
Available-for-Sale, Amortized Cost	3,259,440	$ 3,252,586
Held-to-Maturity, Maturity, Within one year, Amortized Cost	27,128
Held-to-Maturity, Maturity, One to five years, Amortized Cost	33,736
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	75,966
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	388,888
Held-to-Maturity, Amortized Cost	525,718	684,063	$ 695,139
Available-for-Sale, Maturity, Within one year, Fair Value	60,376
Available-for-Sale, Maturity, One to five years, Fair Value	505,586
Available-for-Sale, Maturity, Five to ten years, Fair Value	380,192
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,235,511
Available-for-Sale securities, Fair Value	3,181,665	3,196,207	2,814,338
Held-to-Maturity, Maturity, Within one year, Fair Value	27,341
Held-to-Maturity, Maturity, One to five years, Fair Value	34,432
Held-to-Maturity, Maturity, Five to ten years, Fair Value	77,962
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	384,862
Held-to-Maturity, Fair Value	$ 524,597	$ 727,703	$ 749,363
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.22%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.29%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	3.03%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.64%
Available-for-Sale, Weighted Average Yield	2.62%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	3.75%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.06%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.38%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	3.59%
Held-to-Maturity, Weighted Average Yield	3.74%

Weighted average yield on available-for-sale securities maturing in five to ten years.

No definition available.

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Weighted average yield on available-for-sale securities maturing beyond ten years.

+ References

No definition available.

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

+ References

No definition available.

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

+ References

No definition available.

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- Definition

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.

+ 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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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 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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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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-Name Accounting Standards Codification
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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 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.

+ References