Investment Securities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) Securities	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	$ 45,561,000	$ 20,852,000
Available for sale securities in unrealized loss position | Securities	668
Available for sale securities in portfolio, number | Securities	858
Amortized cost of available for sale securities	$ 2,103,362,000	1,900,684,000
Capitalization of banks, equal to or greater than, in the single-issue trust preferred portfolio	10,000,000,000
Gross realized gains	0		$ 0
Gross realized losses	0		$ 0
Carrying value of securities pledged	1,735,336,000	1,403,565,000
SunTrust Bank [Member]
Schedule of Investments [Line Items]
Amortized cost of trust preferred securities	7,430,000
Royal Bank of Scotland [Member]
Schedule of Investments [Line Items]
Amortized cost of trust preferred securities	976,000
Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale single issue trust preferred securities	5,718,000
Below Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale single issue trust preferred securities	3,024,000
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	5,233,000	2,429,000
Amortized cost of available for sale securities	$ 276,516,000	303,101,000
Percent of portfolio with credit support	75.00%
Investment grade credit rate percentage	1.00%
Number of equity securities other-than-temporarily impaired | Securities	0
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	$ 450,000	4,129,000
Amortized cost of available for sale securities	6,177,000	37,856,000
Trust Preferred Collateralized Debt Obligations [Member] | TRUP CDOs [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	6,177,000
Agency Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	1,427,138,000
Commercial Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	11,936,000	3,309,000
Amortized cost of available for sale securities	513,212,000	457,107,000
Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	24,562,000	9,360,000
Amortized cost of available for sale securities	913,926,000	821,857,000
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	4,306,000	4,969,000
Residential Mortgage-Backed Securities, Non-agency [Member] | Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	250,000
Residential Mortgage-Backed Securities, Non-agency [Member] | Below Investment Grade [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	4,056,000
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Gross unrealized losses on available for sale securities	1,016,000	18,000
Amortized cost of available for sale securities	$ 69,261,000	$ 28,101,000
Number of equity securities other-than-temporarily impaired	69,261,000
Corporate Bonds [Member] | Debt Issuances of Corporations [Member]
Schedule of Investments [Line Items]
Amortized cost of available for sale securities	$ 69,261,000
Corporate Bonds [Member] | Investment Grade [Member]
Schedule of Investments [Line Items]
Percent of corporate securities portfolio	70.00%
Corporate Bonds [Member] | Below Investment Grade [Member]
Schedule of Investments [Line Items]
Percent of corporate securities portfolio	30.00%