Loans - Schedule of Non-Covered and Covered Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	$ 12,622,341	$ 10,996,500
Covered loans receivable	251,120	444,544
Total loans receivable	12,873,461	11,441,044
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	251,120	444,544	664,295
Covered Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	25,508		345,384
Non-Covered Legacy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	9,849,416	9,668,714
Non-Covered Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	2,772,925	1,327,786
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	37,640	220,386
Total loans receivable	8,909,663	7,814,082
Commercial Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	25,508	189,126
Total loans receivable	5,157,670	4,405,133
Commercial Loans [Member] | Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	12,132	31,260
Total loans receivable	3,751,993	3,408,949
Commercial Loans [Member] | Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	37,640	220,386
Commercial Loans [Member] | Covered Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	25,508	189,126
Commercial Loans [Member] | Covered Loans [Member] | Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	12,132	31,260
Commercial Loans [Member] | Non-Covered Legacy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	7,110,864	7,002,198
Commercial Loans [Member] | Non-Covered Legacy Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	3,832,598	3,718,058
Commercial Loans [Member] | Non-Covered Legacy Loans [Member] | Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	3,278,266	3,284,140
Commercial Loans [Member] | Non-Covered Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	1,761,159	591,498
Commercial Loans [Member] | Non-Covered Acquired Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	1,299,564	497,949
Commercial Loans [Member] | Non-Covered Acquired Loans [Member] | Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	461,595	93,549
Residential Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	131,767	128,024
Total loans receivable	1,164,286	1,080,297
Residential Mortgage Loans [Member] | Residential 1-4 Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	131,767	128,024
Total loans receivable	1,101,170	1,048,241
Residential Mortgage Loans [Member] | Construction / Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	63,116	32,056
Residential Mortgage Loans [Member] | Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	131,767	128,024
Residential Mortgage Loans [Member] | Covered Loans [Member] | Residential 1-4 Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	131,767	128,024
Residential Mortgage Loans [Member] | Non-Covered Legacy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	553,815	527,694
Residential Mortgage Loans [Member] | Non-Covered Legacy Loans [Member] | Residential 1-4 Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	490,699	495,638
Residential Mortgage Loans [Member] | Non-Covered Legacy Loans [Member] | Construction / Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	63,116	32,056
Residential Mortgage Loans [Member] | Non-Covered Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	478,704	424,579
Residential Mortgage Loans [Member] | Non-Covered Acquired Loans [Member] | Residential 1-4 Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	478,704	424,579
Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	81,713	96,134
Total loans receivable	2,799,512	2,546,665
Consumer and Other Loans [Member] | Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	81,058	92,430
Total loans receivable	1,858,088	1,601,105
Consumer and Other Loans [Member] | Indirect Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	367,349	397,158
Consumer and Other Loans [Member] | Consumer - Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	655	3,704
Total loans receivable	574,075	548,402
Consumer and Other Loans [Member] | Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	81,713	96,134
Consumer and Other Loans [Member] | Covered Loans [Member] | Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	81,058	92,430
Consumer and Other Loans [Member] | Covered Loans [Member] | Consumer - Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans receivable	655	3,704
Consumer and Other Loans [Member] | Non-Covered Legacy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	2,184,737	2,138,822
Consumer and Other Loans [Member] | Non-Covered Legacy Loans [Member] | Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	1,335,390	1,290,976
Consumer and Other Loans [Member] | Non-Covered Legacy Loans [Member] | Indirect Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	367,077	396,766
Consumer and Other Loans [Member] | Non-Covered Legacy Loans [Member] | Consumer - Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	482,270	451,080
Consumer and Other Loans [Member] | Non-Covered Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	533,062	311,709
Consumer and Other Loans [Member] | Non-Covered Acquired Loans [Member] | Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	441,640	217,699
Consumer and Other Loans [Member] | Non-Covered Acquired Loans [Member] | Indirect Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	272	392
Consumer and Other Loans [Member] | Non-Covered Acquired Loans [Member] | Consumer - Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Covered loans receivable	$ 91,150	$ 93,618