Loans and Allowance for Loan Losses - Changes in Carrying Amount of Covered Acquired Loans and Accretable Yield for Loans Receivable (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Covered loans [Member]
Carrying Amount of Loans, Balance at beginning of period	$ 671,443	$ 809,459
Carrying Amount of Loans, Payments received, net	(150,344)	(193,432)
Carrying Amount of Loans, Accretion	34,633	55,416
Carrying Amount of Loans, Balance at end of period	555,732	671,443
Accretable Discount, Balance at beginning of period	153,137	107,638
Accretable Discount, Accretion	(34,633)	(55,416)
Accretable Discount, Decrease in expected cash flows based on actual cash flow, loan sales and foreclosures assumptions	(9,807)	(18,930)
Accretable Discount, Net transfer from nonaccretable difference to accretable yield	18,967	119,845
Accretable Discount, Balance at end of period	127,664	153,137
Non-covered [Member]
Carrying Amount of Loans, Balance at beginning of period	339,452
Carrying Amount of Loans, Additions		535,489
Carrying Amount of Loans, Payments received, net	(181,287)	(206,306)
Carrying Amount of Loans, Accretion	38,826	10,269
Carrying Amount of Loans, Balance at end of period	196,991	339,452
Accretable Discount, Balance at beginning of period	130,691
Accretable Discount, Additions		132,136
Accretable Discount, Accretion	(38,826)	(22,719)
Accretable Discount, Decrease in expected cash flows based on actual cash flow, loan sales and foreclosures assumptions	(6,180)	(26,630)
Accretable Discount, Net transfer from nonaccretable difference to accretable yield	95,308	47,904
Accretable Discount, Balance at end of period	$ 180,993	$ 130,691