Subordinated Debentures - Additional Information (Detail) - USD ($) $ in Thousands		6 Months Ended
	Feb. 10, 2015	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Subordinated Borrowing [Line Items]
Subordinated debentures, outstanding		$ 121,700
Trust Preferred Securities of the Trusts		118,000
Subordinated debentures, carrying value		117,403	$ 64,950	$ 64,950
Investment in the common equity issued by the trusts		3,700
Aggregate common equity		3,700		$ 1,900
Intervest Trusts [Member]
Subordinated Borrowing [Line Items]
Subordinated debentures, outstanding	$ 56,700
Trust Preferred Securities of the Trusts	55,000
Subordinated debentures, estimated fair value	52,200
Trust common equity	$ 1,700
Intervest Trusts [Member] | Subordinated Debentures [Member]
Subordinated Borrowing [Line Items]
Fair value adjustment, amortization period	8 years
Subordinated debentures, fair value adjustment	$ 4,500
Intervest II [Member]
Subordinated Borrowing [Line Items]
Trust Preferred Securities of the Trusts		$ 15,000
LIBOR plus rate, percentage	2.95%
Interest rate, adjustable quarterly		90-day London Interbank Offered Rates ("LIBOR") plus 2.95%
Maturity date	Sep. 17, 2033
Subordinated debentures, carrying value		$ 14,786
Intervest III [Member]
Subordinated Borrowing [Line Items]
Trust Preferred Securities of the Trusts		$ 15,000
LIBOR plus rate, percentage	2.79%
Interest rate, adjustable quarterly		90-day LIBOR plus 2.79%
Maturity date	Mar. 17, 2034
Subordinated debentures, carrying value		$ 14,679
Intervest IV [Member]
Subordinated Borrowing [Line Items]
Trust Preferred Securities of the Trusts		$ 15,000
LIBOR plus rate, percentage	2.40%
Interest rate, adjustable quarterly		90-day LIBOR plus 2.40%
Maturity date	Sep. 20, 2034
Subordinated debentures, carrying value		$ 14,036
Intervest V [Member]
Subordinated Borrowing [Line Items]
Trust Preferred Securities of the Trusts		$ 10,000
LIBOR plus rate, percentage	1.65%
Interest rate, adjustable quarterly		90-day LIBOR plus 1.65%
Maturity date	Dec. 15, 2036
Subordinated debentures, carrying value		$ 8,952
Debentures [Member]
Subordinated Borrowing [Line Items]
Interest payment terms		Period not to exceed five consecutive years