Investment Securities Held-to-Maturity - Additional Information (Detail)	3 Months Ended
	Mar. 31, 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	$ 1,499,836,000	$ 1,441,059,000
Securities held to maturity at floating rate or adjustable rate	$ 120,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	4 years
Number of securities, temporarily impaired for less than 12 months | Security	12	56
Number of securities, temporarily impaired for 12 months or longer | Security	304	315
Number of securities, temporarily impaired, total | Security	487	475
Held-to-Maturity Securities [Member] | U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life	$ 116,160,000
Federal Home Loan Bank [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank	$ 305,776,000	$ 291,190,000