Investment Securities - Additional Information (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) Securities	Dec. 31, 2019 USD ($)
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	$ 21,178	$ 18,540
Available for sale securities in unrealized loss position | Securities	77
Available for sale securities in portfolio, number | Securities	982
Amortized cost of available for sale securities	$ 2,730,350	2,426,924
Capitalization of banks, equal to or greater than, in the single-issue trust preferred portfolio	10,000,000
Equity securities at estimated fair value	9,875	8,894
Carrying value of securities pledged	$ 2,150,574	1,540,717
Percentage Of Guarantee Repayment By Government	97.00%
Accrued interest receivable	$ 11,275	9,890
Below Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale single issue trust preferred securities	11,492
Split Rated Bonds Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale single issue trust preferred securities	978
Unrated Bonds Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale single issue trust preferred securities	5,741
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	236	3,296
Amortized cost of available for sale securities	$ 502,293	272,014
Percent of portfolio with credit support	57.00%
Agency Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	$ 1,410,376
Commercial Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	99	2,807
Amortized cost of available for sale securities	591,303	609,461
Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	51	1,246
Amortized cost of available for sale securities	819,073	826,857
Residential Mortgage-Backed Securities, Non-agency [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	0	0
Amortized cost of available for sale securities	$ 61,954	3,429
Percentage of available for sale securities at amortized cost unrated	7.00%
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Recognized in Earnings	$ 0
Residential Mortgage-Backed Securities, Non-agency [Member] | AAA [Member]
Schedule of Investments [Line Items]
Percentage of available for sale securities at amortized cost rated	93.00%
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	$ 1,702	1,342
Amortized cost of available for sale securities	11,895	6,045
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	1,336	0
Amortized cost of available for sale securities	$ 365,934	$ 348,405
Corporate Bonds [Member] | Investment Grade [Member]
Schedule of Investments [Line Items]
Percent of corporate securities portfolio	88.00%
Corporate Bonds [Member] | Unrated Bonds Investment Grade [Member]
Schedule of Investments [Line Items]
Percent of corporate securities portfolio	12.00%