Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Maturity period of highly liquid debt instruments	Three months or less
Available-for-sale securities transferred to held-to-maturity securities		$ 536,010,000
Unrealized net loss on securities reclassified from available-for-sale to held-to-maturity, Gross		13,300,000
Loans past due, minimum period	90 days
Non-accrual business and commercial real estate loans	$ 250,000
Delinquency period of losses on mortgage loan to be deemed uncollectible	120 days
Mortgage loans sold on real estate, final maturity period	15 years
Delinquency period of mortgage loans	90 days
Computer Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets	3 years
Furniture, Fixtures and Other Electronic Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets	5 years
Building Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets	10 years
Buildings [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets	30 years