Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 308,175us-gaap_TradingSecurities	$ 376,131us-gaap_TradingSecurities
Investment securities	9,156,932us-gaap_InvestmentsFairValueDisclosure	4,531,786us-gaap_InvestmentsFairValueDisclosure
Real estate loans held for sale	742,249	468,650
Other assets	92,129us-gaap_OtherAssetsFairValueDisclosure	123,568us-gaap_OtherAssetsFairValueDisclosure
Total assets	10,299,485us-gaap_AssetsFairValueDisclosure	5,500,135us-gaap_AssetsFairValueDisclosure
Trading account liabilities	203,464	249,797
Other liabilities	8,596	4,392
Total liabilities	212,060us-gaap_LiabilitiesFairValueDisclosure	254,189us-gaap_LiabilitiesFairValueDisclosure
U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	161,947	37,776
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	8,198	10,811
Government Issued or Guaranteed [Member] \| Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	8,731,123	4,165,086
Privately Issued Mortgage-Backed Securities [Member] \| Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	103	1,850
Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	50,316	63,083
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	121,488	120,085
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	83,757	133,095
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	51,416	51,386
Investment securities	64,841	82,450
Total assets	116,257	133,836
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	64,841	82,450
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	256,759	324,745
Investment securities	9,041,672	4,384,403
Real estate loans held for sale	742,249	468,650
Other assets	74,733	115,952
Total assets	10,115,413	5,293,750
Trading account liabilities	203,464	249,797
Other liabilities	8,547	717
Total liabilities	212,011	250,514
Significant Observable Inputs (Level 2) [Member] \| U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	161,947	37,776
Significant Observable Inputs (Level 2) [Member] \| Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	8,198	10,811
Significant Observable Inputs (Level 2) [Member] \| Government Issued or Guaranteed [Member] \| Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	8,731,123	4,165,086
Significant Observable Inputs (Level 2) [Member] \| Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	121,488	120,085
Significant Observable Inputs (Level 2) [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	18,916	50,645
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	50,419	64,933
Other assets	17,396	7,616
Total assets	67,815	72,549
Other liabilities	49	3,675
Total liabilities	49	3,675
Significant Unobservable Inputs (Level 3) [Member] \| Privately Issued Mortgage-Backed Securities [Member] \| Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	103	1,850
Significant Unobservable Inputs (Level 3) [Member] \| Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	$ 50,316	$ 63,083

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

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Fair value of financial and nonfinancial obligations.

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- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

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- Definition

Fair value portion of other assets.

+ References

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Fair value portion of other liabilities.

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- Definition

Fair value portion of liabilities pertaining to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include, but are not limited to, short positions in securities, derivatives and commodities.

+ References

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- Definition

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

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