Fair Value Measurement of Assets and Liabilities - Recurring and Non-Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities
Available for sale	$ 1,843,514	$ 1,493,905
Non-recurring fair value measurements
Unpaid principal balances of loans held for sale	8,300	14,800
U.S. Treasury securities
Investment securities
Available for sale	48,906	49,642
U.S. government agency securities
Investment securities
Available for sale	40,590	42,505
Obligations of states and political subdivisions
Investment securities
Available for sale	221,174	112,884
Residential mortgage-backed securities
Investment securities
Available for sale	1,474,075	1,223,295
Trust preferred securities
Investment securities
Available for sale	3,183	3,214
Corporate and other debt securities
Investment securities
Available for sale	55,586	51,164
Equity securities
Investment securities
Available for sale		11,201
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements
Investment securities
Available for sale	56,596	58,807
Loans held for sale	0	0
Other assets	0	0
Total assets	56,596	58,807
Liabilities
Other liabilities	0	0
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| U.S. Treasury securities
Investment securities
Available for sale	48,906	49,642
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| U.S. government agency securities
Investment securities
Available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| Obligations of states and political subdivisions
Investment securities
Available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| Residential mortgage-backed securities
Investment securities
Available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| Trust preferred securities
Investment securities
Available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| Corporate and other debt securities
Investment securities
Available for sale	7,690	7,783
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring fair value measurements \| Equity securities
Investment securities
Available for sale		1,382
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Non-recurring fair value measurements
Non-recurring fair value measurements
Collateral dependent impaired loans	0	0
Loan servicing rights	0	0
Foreclosed assets	0	0
Total	0	0
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements
Investment securities
Available for sale	1,780,420	1,427,738
Loans held for sale	8,449	15,119
Other assets	25,982	26,417
Total assets	1,814,851	1,469,274
Liabilities
Other liabilities	30,119	24,330
Total liabilities	30,119	24,330
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| U.S. Treasury securities
Investment securities
Available for sale	0	0
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| U.S. government agency securities
Investment securities
Available for sale	40,590	42,505
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| Obligations of states and political subdivisions
Investment securities
Available for sale	221,174	112,884
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| Residential mortgage-backed securities
Investment securities
Available for sale	1,467,577	1,215,935
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| Trust preferred securities
Investment securities
Available for sale	3,183	3,214
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| Corporate and other debt securities
Investment securities
Available for sale	47,896	43,381
Significant Other Observable Inputs (Level 2) \| Recurring fair value measurements \| Equity securities
Investment securities
Available for sale		9,819
Significant Other Observable Inputs (Level 2) \| Non-recurring fair value measurements
Non-recurring fair value measurements
Collateral dependent impaired loans	0	0
Loan servicing rights	0	0
Foreclosed assets	0	0
Total	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements
Investment securities
Available for sale	6,498	7,360
Loans held for sale	0	0
Other assets	0	0
Total assets	6,498	7,360
Liabilities
Other liabilities	0	0
Total liabilities	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| U.S. Treasury securities
Investment securities
Available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| U.S. government agency securities
Investment securities
Available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| Obligations of states and political subdivisions
Investment securities
Available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| Residential mortgage-backed securities
Investment securities
Available for sale	6,498	7,360
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| Trust preferred securities
Investment securities
Available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| Corporate and other debt securities
Investment securities
Available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Recurring fair value measurements \| Equity securities
Investment securities
Available for sale		0
Significant Unobservable Inputs (Level 3) \| Non-recurring fair value measurements
Non-recurring fair value measurements
Collateral dependent impaired loans	40,872	48,373
Loan servicing rights	2,831	5,350
Foreclosed assets	923	3,472
Total	44,626	57,195
Fair Value \| Recurring fair value measurements
Investment securities
Available for sale	1,843,514	1,493,905
Loans held for sale	8,449	15,119
Other assets	25,982	26,417
Total assets	1,877,945	1,535,441
Liabilities
Other liabilities	30,119	24,330
Total liabilities	30,119	24,330
Fair Value \| Recurring fair value measurements \| U.S. Treasury securities
Investment securities
Available for sale	48,906	49,642
Fair Value \| Recurring fair value measurements \| U.S. government agency securities
Investment securities
Available for sale	40,590	42,505
Fair Value \| Recurring fair value measurements \| Obligations of states and political subdivisions
Investment securities
Available for sale	221,174	112,884
Fair Value \| Recurring fair value measurements \| Residential mortgage-backed securities
Investment securities
Available for sale	1,474,075	1,223,295
Fair Value \| Recurring fair value measurements \| Trust preferred securities
Investment securities
Available for sale	3,183	3,214
Fair Value \| Recurring fair value measurements \| Corporate and other debt securities
Investment securities
Available for sale	55,586	51,164
Fair Value \| Recurring fair value measurements \| Equity securities
Investment securities
Available for sale		11,201
Fair Value \| Non-recurring fair value measurements
Non-recurring fair value measurements
Collateral dependent impaired loans	40,872	48,373
Loan servicing rights	2,831	5,350
Foreclosed assets	923	3,472
Total	$ 44,626	$ 57,195

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value of asset after deduction of liability.

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Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.

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Fair value of financial and nonfinancial obligations.

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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Fair value of the collateral dependent impaired loans.

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