Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Cash and cash equivalents, interest-bearing deposits in other financial institutions	$ 537,700,000	$ 564,200,000
Cash and cash equivalents, interest-bearing deposits in other financial institutions, Federal Reserve Bank of New York	119,200,000	135,200,000
Cash and cash equivalents, federal funds sold	4,600,000	6,800,000
Cash and cash equivalents, outstanding reverse repurchase agreements	250,000,000	250,000,000
Minimum amount required to be maintained as total reserves with the Federal Reserve Bank of New York	158,300,000	129,500,000
Depreciation and amortization	31,497,000	27,792,000	$ 28,092,000
Bank-owned life insurance	931,627,000	915,156,000
Bank-owned life insurance income	27,541,000	27,150,000	$ 29,938,000
Purchase of bank-owned life insurance policy	0	0
Other real estate owned	39,882,000	94,004,000
Other real estate owned, covered by FDIC loss sharing agreements	$ 25,817,000	$ 32,048,000
Shares available for grant	12,233,512
Building
Significant Accounting Policies [Line Items]
Premises, furniture, fixtures, and equipment, estimated useful lives (in years)	20 years
Options and Restricted Stock
Significant Accounting Policies [Line Items]
Shares available for grant	12,233,512
Two Thousand Twelve Incentive Plan
Significant Accounting Policies [Line Items]
Shares transferred	1,030,673
AmTrust Bank
Significant Accounting Policies [Line Items]
Loss sharing agreements, expiration	2019-12
Desert Hills Bank
Significant Accounting Policies [Line Items]
Loss sharing agreements, expiration	2020-03
Maximum | Furniture and Fixtures
Significant Accounting Policies [Line Items]
Premises, furniture, fixtures, and equipment, estimated useful lives (in years)	10 years
Maximum | Equipment
Significant Accounting Policies [Line Items]
Premises, furniture, fixtures, and equipment, estimated useful lives (in years)	10 years
Minimum | Furniture and Fixtures
Significant Accounting Policies [Line Items]
Premises, furniture, fixtures, and equipment, estimated useful lives (in years)	3 years
Minimum | Equipment
Significant Accounting Policies [Line Items]
Premises, furniture, fixtures, and equipment, estimated useful lives (in years)	3 years
Core deposit intangibles | Maximum
Significant Accounting Policies [Line Items]
Core deposit intangible, estimated useful lives (in years)	10 years