Investment Securities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2017 USD ($) Security Segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Investment [Line Items]
Trading securities that consist of mutual funds	$ 5,235,000	$ 4,838,000	$ 4,982,000
Number of segments | Segment	2
OTTI losses on securities	$ 0	0
Number of securities in security portfolio | Security	1,615
Number of securities in unrealized loss position | Security	296
Unrealized losses	$ 36,721,000		73,936,000
Former Directors and Executives [Member]
Investment [Line Items]
Trading securities that consist of mutual funds	$ 5,200,000		5,000,000
Pooled Trust Preferred Securities, Not Subject to FASB ASC 325-10 [Member]
Investment [Line Items]
Number of trust preferred securities | Security	2
Securities portfolio at fair value	$ 7,800,000
Unrealized losses	$ 6,100,000
Pooled Trust Preferred Securities, Subject To FASB ASC 325-10 [Member]
Investment [Line Items]
Number of trust preferred securities | Security	1
Securities portfolio at fair value	$ 300,000
Unrealized losses	$ 2,600,000
Pooled Trust Preferred Securities [Member]
Investment [Line Items]
Number of trust preferred securities | Security	3
Securities portfolio at fair value	$ 8,100,000
Unrealized losses	8,700,000
Unrealized losses	8,710,000		$ 8,892,000
OTTI losses	$ 0	$ 0