Loans and Allowance for Loan Losses - Schedule of Accretable Yield of Noncovered PCI Loans, or Income Expected to be Collected (Detail) - Purchased Credit Impaired Loans [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	$ 33,603	$ 45,310	$ 62,533
New loans purchased	1,556	3,217	1,812
Accretion of income	(15,217)	(23,447)	(35,526)
Reclassifications from (to) nonaccretable difference	7,614	10,589	14,189
Disposals/other adjustments	279	(2,066)	2,302
Balance at end of period	$ 27,835	$ 33,603	$ 45,310