Time Deposits - Summary of Large time Deposits and time Deposits Meet or Exceed Current FDIC Insurance Limit for Periods Presented (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments Schedule [Abstract]
Time deposits of $100,000 or more	$ 319,811	$ 328,363
Time deposits of $250,000 or more	$ 122,391	$ 122,211

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=50488274&loc=d3e64164-112818

Name:	us-gaap_TimeDepositsAtOrAboveFDICInsuranceLimit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant