Activity in Allowance for Losses on Covered Loans (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015
Am Trust Bank and Desert Hills Bank
Valuation Allowance [Line Items]
Allowance associated on loans acquired	$ 14.2	$ 14.2

Amount of addition to allowance reserve for credit loss arising from purchase of loan and lease portfolios.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_AllowanceForLoanAndLeaseLossesLoansAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration