Business Combinations - Schedule of Changes in Accretable Discounts Related Acquired Non-Covered Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Schedule Of Covered Loans Acquired Accretable Discounts [Line Items]
Balance, beginning of year	$ 15,578	$ 25,493	$ 25,493
Accretion	(4,466)	(9,767)	(22,188)
Transfers between non-accretable and accretable discounts, net	1,853	365	12,273
Balance, end of year	12,965	16,091	15,578
Non-Covered Loans [Member]
Schedule Of Covered Loans Acquired Accretable Discounts [Line Items]
Balance, beginning of year	674,239	448,753	448,753
Additions due to acquisitions			279,441
Accretion	(3,111)	(1,023)	(9,745)
Balance, end of year	643,092	437,269	674,239
Non-Covered Loans [Member] | Discount Accretion [Member]
Schedule Of Covered Loans Acquired Accretable Discounts [Line Items]
Balance, beginning of year	25,716	26,189	26,189
Additions due to acquisitions			7,799
Accretion	(3,111)	(1,023)	(9,745)
Transfers between non-accretable and accretable discounts, net	(2,376)	2,680	1,473
Balance, end of year	$ 20,229	$ 27,846	$ 25,716