Borrowings - Schedule of Borrowings (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Disclosure [Abstract]
Securities sold under agreements with customers to repurchase	$ 378,460	$ 320,450	$ 339,660
Federal funds purchased	2,300	10,550	11,775
Total	$ 380,760	$ 331,000	$ 351,435

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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