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	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale securities:
Assets measured at fair value on a recurring basis	$ 2,109,893	$ 1,759,905
Assets
Assets measured at fair value on a nonrecurring basis	9,594	11,529
Specific reserves related to impaired loans	4,842	1,342
Derivative Assets
Available-for-sale securities:
Assets measured at fair value on a recurring basis	10,938	7,552
Loans held for sale – fair value option
Available-for-sale securities:
Assets measured at fair value on a recurring basis	1,680,010	1,335,668
Derivative Liabilities
Liabilities
Derivatives	16,951	9,716
Impaired loans
Assets
Assets measured at fair value on a nonrecurring basis	6,619	2,380
Specific reserves related to impaired loans	4,842	1,342
Other real estate owned
Assets
Assets measured at fair value on a nonrecurring basis	2,975	9,149
Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	365,760	377,311
Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets measured at fair value on a recurring basis	35,322	15,104
Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	17,863	24,270
Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale securities:
Assets measured at fair value on a recurring basis	17,863	24,270
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Assets
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Loans held for sale – fair value option
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Liabilities
Liabilities
Derivatives	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Impaired loans
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other real estate owned
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	17,863	24,270
Significant Other Observable Inputs (Level 2)
Available-for-sale securities:
Assets measured at fair value on a recurring basis	2,091,960	1,735,592
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) | Derivative Assets
Available-for-sale securities:
Assets measured at fair value on a recurring basis	10,868	7,509
Significant Other Observable Inputs (Level 2) | Loans held for sale – fair value option
Available-for-sale securities:
Assets measured at fair value on a recurring basis	1,680,010	1,335,668
Significant Other Observable Inputs (Level 2) | Derivative Liabilities
Liabilities
Derivatives	16,951	9,716
Significant Other Observable Inputs (Level 2) | Impaired loans
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) | Other real estate owned
Assets
Assets measured at fair value on a nonrecurring basis	0	0
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	365,760	377,311
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets measured at fair value on a recurring basis	35,322	15,104
Significant Other Observable Inputs (Level 2) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3)
Available-for-sale securities:
Assets measured at fair value on a recurring basis	70	43
Assets
Assets measured at fair value on a nonrecurring basis	9,594	11,529
Significant Unobservable Inputs (Level 3) | Derivative Assets
Available-for-sale securities:
Assets measured at fair value on a recurring basis	70	43
Significant Unobservable Inputs (Level 3) | Loans held for sale – fair value option
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Derivative Liabilities
Liabilities
Derivatives	0	0
Significant Unobservable Inputs (Level 3) | Impaired loans
Assets
Assets measured at fair value on a nonrecurring basis	6,619	2,380
Significant Unobservable Inputs (Level 3) | Other real estate owned
Assets
Assets measured at fair value on a nonrecurring basis	2,975	9,149
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Mortgage-backed securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Corporate notes
Available-for-sale securities:
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Available-for-sale Securities | Equity securities
Available-for-sale securities:
Assets measured at fair value on a recurring basis	$ 0	$ 0