Business Combinations - Rollforward of 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
Purchased Non Covered Loans [Line Items]
Balance, beginning of year	$ 15,578	$ 25,493	$ 25,493
Accretion	4,466	9,767	22,188
Transfers to purchased non-covered other real estate owned	(1,094)	(68)
Balance, end of year	12,965	16,091	15,578
Non-Covered Loans [Member]
Purchased Non Covered Loans [Line Items]
Balance, beginning of year	674,239	448,753	448,753
Charge-offs, net of recoveries	(244)		(84)
Additions due to acquisitions			279,441
Accretion	3,111	1,023	9,745
Transfers to purchased non-covered other real estate owned	(1,094)	(68)	(4,160)
Transfer from covered loans due to loss share expiration			15,475
Payments received	(32,920)	(12,439)	(74,931)
Balance, end of year	$ 643,092	$ 437,269	$ 674,239