Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of Activity in the Allowance for Covered Loan Losses) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Allowance for Covered Loan Losses [Roll Forward]
Balance at beginning of period	$ 2,131	$ 10,092
Provision for covered loan losses before benefit attributable to FDIC loss share agreements	(529)	(7,121)
Benefit attributable to FDIC loss share agreements	423	5,697
Net provision for covered loan losses	(106)	(1,424)
Decrease in FDIC indemnification asset	(423)	(5,697)
Loans charged-off	(237)	(2,864)
Recoveries of loans charged-off	513	3,340
Net recoveries	276	476
Balance at end of period	$ 1,878	$ 3,447