Loans - Additional Information (Detail) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Investment	Sep. 30, 2014 USD ($) Investment	Dec. 31, 2014 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans		$ 34,124,905		$ 34,124,905		$ 33,004,361
Number of loans classified as a non-accrual TDRs | Investment				9
Covered loans		2,149,055		$ 2,149,055		2,428,622
Recovery of loan losses		(9,028)	$ (3,945)	(8,687)	$ (18,387)
FDIC indemnification expense		6,800	3,200	4,300	$ 14,700
Financing Receivable Troubled Debt Restructurings Rate Reductions
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates				21,200
Financing Receivable Troubled Debt Restructurings Forbearance Of Arrears
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates				5,300
Purchased Credit Impaired loans
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans	[1]	12,976		12,976
Purchased credit-impaired loans outstanding		14,800		14,800
Mortgage Receivable
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans		32,675,646		32,675,646		31,863,197
Unpaid principal balance of serviced loans		23,900,000		23,900,000		22,400,000
Acquisition, Development and Construction
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans		294,768		294,768		258,116
Number of loans classified as a non-accrual TDRs | Investment					1
Loan classified as non accrual TDRs			935		$ 935
Commercial and Industrial
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans	[2]	1,410,214		1,410,214		1,109,221	[3]
Number of loans classified as a non-accrual TDRs | Investment					1
Loan classified as non accrual TDRs			499		$ 499
Multifamily Mortgage Loans
Financing Receivable, Recorded Investment [Line Items]
Number of loans classified as a non-accrual TDRs | Investment					1
Loan classified as non accrual TDRs			316		$ 316
Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Non-Covered Loans		7,642,994		$ 7,642,994		7,634,320
Number of loans classified as a non-accrual TDRs | Investment				3	1
Loan classified as non accrual TDRs			2,100		$ 2,100
Covered Loans
Financing Receivable, Recorded Investment [Line Items]
Current		2,000,000		$ 2,000,000
Recovery of loan losses		(8,516)	$ (3,945)	(5,433)	$ (18,387)
Covered Loans | Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		142,176		142,176		157,889
Covered Loans | Performing Financial Instruments | Financing Receivable Recorded Investment Thirty to Eighty Nine Days Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		38,282		38,282
Covered Loans | Performing Financial Instruments | Loans 90 Days Or More Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		142,176		142,176
Non-officer directors
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to non-officer directors		126,600		126,600		129,500
Am Trust Bank and Desert Hills Bank
Financing Receivable, Recorded Investment [Line Items]
Total loans, net		$ 2,600,000		$ 2,600,000		$ 2,900,000

[1] Includes $941,000 of multi-family loans; $9.9 million of commercial real estate loans; $1.0 million of acquisition, development, and construction loans; $965,000 of commercial and industrial loans; and $163,000 of other loans that were included in "Covered loans" at December 31, 2014.

[2] Includes lease financing receivables, all of which were classified as "pass."

[3] Includes lease financing receivables, all of which were current.