Business Combinations - Rollforward of Acquired Non-Covered Loans (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Purchased Non Covered Loans [Line Items]
Balance, beginning of year	$ 15,578	$ 25,493	$ 25,493
Accretion	6,251	15,432	22,188
Transfers to purchased non-covered other real estate owned	(2,039)	(1,425)
Transfer from covered loans due to loss-share expiration	84
Balance, end of year	12,060	15,911	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	(470)		(84)
Additions due to acquisitions	195,818	279,441	279,441
Accretion	5,388	3,635	9,745
Transfers to purchased non-covered other real estate owned	(2,039)	(1,425)	(4,160)
Transfer from covered loans due to loss-share expiration	15,462		15,475
Payments received	(80,085)	(28,273)	(74,931)
Balance, end of year	$ 808,313	$ 702,131	$ 674,239