Disclosures about Fair Value of Financial Instruments - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring and Nonrecurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	$ 2,081,895	$ 1,759,905
Assets
Assets, Fair Value Disclosure, Nonrecurring	5,489	11,529
Specific reserves related to impaired loans	3,369	1,342
Derivative Assets
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	8,810	7,552
Loans held for sale – fair value option
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	1,676,891	1,335,668
Derivative Liabilities
Liabilities
Liabilities, Fair Value Disclosure, Recurring	12,938	9,716
Impaired loans
Assets
Assets, Fair Value Disclosure, Nonrecurring	4,943	2,380
Specific reserves related to impaired loans	3,369	1,342
Other real estate owned
Assets
Assets, Fair Value Disclosure, Nonrecurring	546	9,149
Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	358,717	377,311
Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	15,172	15,104
Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	22,305	24,270
Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	22,305	24,270
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Assets
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Loans held for sale – fair value option
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Liabilities
Liabilities
Liabilities, Fair Value Disclosure, Recurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Impaired loans
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other real estate owned
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	22,305	24,270
Significant Other Observable Inputs (Level 2)
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	2,059,503	1,735,592
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Significant Other Observable Inputs (Level 2) | Derivative Assets
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	8,723	7,509
Significant Other Observable Inputs (Level 2) | Loans held for sale – fair value option
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	1,676,891	1,335,668
Significant Other Observable Inputs (Level 2) | Derivative Liabilities
Liabilities
Liabilities, Fair Value Disclosure, Recurring	12,938	9,716
Significant Other Observable Inputs (Level 2) | Impaired loans
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Significant Other Observable Inputs (Level 2) | Other real estate owned
Assets
Assets, Fair Value Disclosure, Nonrecurring	0	0
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	358,717	377,311
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	15,172	15,104
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Significant Unobservable Inputs (Level 3)
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	87	43
Assets
Assets, Fair Value Disclosure, Nonrecurring	5,489	11,529
Significant Unobservable Inputs (Level 3) | Derivative Assets
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	87	43
Significant Unobservable Inputs (Level 3) | Loans held for sale – fair value option
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Significant Unobservable Inputs (Level 3) | Derivative Liabilities
Liabilities
Liabilities, Fair Value Disclosure, Recurring	0	0
Significant Unobservable Inputs (Level 3) | Impaired loans
Assets
Assets, Fair Value Disclosure, Nonrecurring	4,943	2,380
Significant Unobservable Inputs (Level 3) | Other real estate owned
Assets
Assets, Fair Value Disclosure, Nonrecurring	546	9,149
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	0	0
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets, Fair Value Disclosure, Recurring	$ 0	$ 0