Investment Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | security	345		152
Fair value of investments held to maturity pledged as collateral	$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years	8 years 7 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | security	548		327
Amortized cost	$ 1,890,335,000		$ 1,511,034,000
Available for sale	1,843,514,000		1,493,905,000
Gross unrealized losses	49,361,000		$ 22,158,000
Fair value of securities available for sale pledged as collateral	$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years	8 years 8 months
Other than temporary impairment losses recognized in earnings	$ 0	$ 0
Losses on securities transactions, net	(765,000)	$ (23,000)
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	7,400,000
Available for sale	$ 6,500,000
Single Issuer Trust Preferred Securities | Non-Rated
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | security	4