Junior Subordinated Debentures Issued to Capital Trusts - Schedule of Outstanding Junior Subordinated Debentures and Related Trust Preferred Securities Issued by Each Trust (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2017
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Carrying value	$ 41,774,000	$ 41,577,000
GCB Capital Trust III
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Effective interest rate			6.96%
GCB Capital Trust III | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage	1.40%
Junior Subordinated Debentures | GCB Capital Trust III
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Carrying value	$ 24,743,000	24,777,000
Contractual principal balance	24,743,000	$ 24,743,000
Junior Subordinated Debentures | GCB Capital Trust III | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage		1.40%
Junior Subordinated Debentures | State Bancorp Capital Trust I
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Carrying value	8,824,000	$ 8,724,000
Contractual principal balance	10,310,000	$ 10,310,000
Junior Subordinated Debentures | State Bancorp Capital Trust I | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage		3.45%
Junior Subordinated Debentures | State Bancorp Capital Trust II
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Carrying value	8,207,000	$ 8,076,000
Contractual principal balance	$ 10,310,000	$ 10,310,000
Junior Subordinated Debentures | State Bancorp Capital Trust II | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage		2.85%
Trust Preferred Securities | GCB Capital Trust III
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Face value		$ 24,000,000
Trust Preferred Securities | GCB Capital Trust III | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage	1.40%	1.40%
Trust Preferred Securities | State Bancorp Capital Trust I
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Face value		$ 10,000,000
Trust Preferred Securities | State Bancorp Capital Trust I | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage	3.45%	3.45%
Trust Preferred Securities | State Bancorp Capital Trust II
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Face value		$ 10,000,000
Trust Preferred Securities | State Bancorp Capital Trust II | 3-month LIBOR
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Annual interest rate spread, percentage	2.85%	2.85%