Securities - Amortized Cost and Fair Value of Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	$ 1,792,947	$ 1,631,761
Securities available for sale, Gross Unrealized Gains	36,193	32,543
Securities available for sale, Gross Unrealized Losses	3,532	4,139
Securities Available for Sale, Fair Value	1,825,608us-gaap_AvailableForSaleSecurities	1,660,165us-gaap_AvailableForSaleSecurities
U.S. Treasury and Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	100,182	300,207
Securities available for sale, Gross Unrealized Gains		372
Securities available for sale, Gross Unrealized Losses	766	71
Securities Available for Sale, Fair Value	99,416	300,508
Municipal Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	12,734	13,995
Securities available for sale, Gross Unrealized Gains	231	186
Securities available for sale, Gross Unrealized Losses		5
Securities Available for Sale, Fair Value	12,965	14,176
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	3,500	3,500
Securities Available for Sale, Fair Value	3,500	3,500
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	2,512	8,673
Securities available for sale, Gross Unrealized Gains	322	891
Securities available for sale, Gross Unrealized Losses	8	11
Securities Available for Sale, Fair Value	2,826	9,553
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	1,441,474	1,217,293
Securities available for sale, Gross Unrealized Gains	35,262	31,094
Securities available for sale, Gross Unrealized Losses	1,947	2,823
Securities Available for Sale, Fair Value	1,474,789	1,245,564
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	232,545	88,093
Securities available for sale, Gross Unrealized Gains	378
Securities available for sale, Gross Unrealized Losses	811	1,229
Securities Available for Sale, Fair Value	$ 232,112	$ 86,864

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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-Publisher FASB

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