Investment Securities Held-to-Maturity - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule of Held-to-maturity Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,692,033,000	$ 1,441,059,000
Securities held to maturity at floating rate or adjustable rate	111,000
Sales of held-to-maturity securities	1,194,000
Commercial Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Gross realised gains on sale of held to maturity securities	48,000
Sales of held-to-maturity securities	$ 1,194,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	3 years 7 months 6 days
Number of securities, temporarily impaired for less than 12 months | Security	44	56
Number of securities, temporarily impaired for 12 months or longer | Security	112	315
Number of securities, temporarily impaired, total | Security	505	475
Held-to-Maturity Securities [Member] | U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life	$ 66,196,000
Federal Home Loan Bank [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank	$ 427,425,000	$ 291,190,000