Loans, Allowance for Loan and Lease Losses, and Reserve for Off-Balance Sheet Credit Commitments (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	$ 22,536,584		$ 20,337,206	$ 19,347,988
Less: Allowance for loan and lease losses	(317,157)	$ (316,922)	(310,149)	(312,703)	$ (311,276)	$ (302,584)
Loans and leases, excluding acquired impaired loans, net	22,219,427		20,027,057
Acquired impaired loans	399,527		510,979
Less: Allowance for loan losses	(5,648)	(8,075)	(8,608)	(9,368)	(9,103)	(15,922)
Acquired impaired loans, net	393,879		502,371
Total loans and leases	22,936,111		20,848,185
Net loans and leases	22,613,306		20,529,428
Net deferred costs (fees)	3,500		200
FDIC indemnification asset	28,164		50,511
FDIC clawback liability	16,608
Acquired impaired loans covered under the FDIC loss-sharing agreements	377,100
Acquired impaired loans not covered under the FDIC loss-sharing agreements	22,400
Imperial Capital Bank
Accounts notes and loans receivable
Acquired impaired loans	359,723		456,177
FDIC indemnification asset	25,960
1st Pacific Bank
Accounts notes and loans receivable
Acquired impaired loans	18,981		23,895
FDIC indemnification asset	760
FDIC clawback liability	13,395
Sun West Bank
Accounts notes and loans receivable
Acquired impaired loans	6,356		9,353
FDIC clawback liability	2,801
Nevada Commerce Bank
Accounts notes and loans receivable
Acquired impaired loans	14,467		21,554
FDIC indemnification asset	1,444
FDIC clawback liability	412
Commercial
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	10,253,592		9,360,976
Acquired impaired loans	$ 994		1,969
Sharing of recoveries with FDIC on expiration date (as a percent)	80.00%
Sharing of recoveries with FDIC on termination date (as a percent)	80.00%
Commercial real estate mortgages
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	$ 3,947,746		3,539,703	3,565,188
Less: Allowance for loan and lease losses	(48,506)	(48,649)	(44,745)	(50,989)	(50,651)	(50,678)
Acquired impaired loans	376,335		481,689
Residential mortgages
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	5,709,272		5,106,803	5,023,213
Less: Allowance for loan and lease losses	(9,848)	(9,660)	(10,296)	(11,467)	(10,296)	(11,540)
Acquired impaired loans	3,391		4,455
Real estate construction
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	932,422		710,224	585,232
Less: Allowance for loan and lease losses	(11,577)	(10,652)	(9,115)	(8,745)	(7,191)	(6,351)
Acquired impaired loans	16,042		18,790
Home equity loans and lines of credit
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	799,711		785,796	759,258
Less: Allowance for loan and lease losses	(5,278)	(5,404)	(6,609)	(7,028)	(6,575)	(6,677)
Acquired impaired loans	2,612		3,820
Installment
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	206,156		184,613	178,803
Less: Allowance for loan and lease losses	(2,290)	$ (2,208)	(2,228)	$ (2,384)	$ (2,284)	$ (1,842)
Acquired impaired loans	153		256
Lease financing
Accounts notes and loans receivable
Loans and leases, excluding acquired impaired loans	$ 687,685		$ 649,091