Goodwill and Other Intangible Assets - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Goodwill, Other Changes	$ 0
Goodwill impairment	0	0	0
Impairment of core deposits and intangibles	0	0	0
Amortization expense - other intangible assets	9,900,000	8,300,000	9,800,000
Core Deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	11 years
Core Deposits [Member] | State Bancorp, Inc. [Member]
Finite-Lived Intangible Assets [Line Items]
Core deposits intangibles from acquisition			11,500,000
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	18 years
Residential Mortgage Loans [Member]
Finite-Lived Intangible Assets [Line Items]
Aggregate principal balances of mortgage loans serviced	2,300,000,000	2,500,000,000	1,800,000,000
SBA Loans [Member]
Finite-Lived Intangible Assets [Line Items]
Total amount of loans serviced by bank	$ 26,900,000	$ 28,400,000	$ 30,400,000