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Significant Accounting Policies and Basis of Presentation - Narrative (Detail) |  | 12 Months Ended |
--- | --- | --- | ---
Jan. 01, 2020 USD ($) | Dec. 31, 2020 USD ($) CompensationPlan position contract Segment Swap | Dec. 31, 2019 USD ($) contract Swap | Dec. 31, 2018 USD ($) | Dec. 31, 2014 | Dec. 31, 2017 USD ($)
Derivative [Line Items] |  |  |  |  |  |
Average reserve balances required to be maintained with federal reserve bank |  | $ 0 | $ 69,100,000 |  |  |
Available-for-sale debt securities, accrued interest |  | 4,200,000 |  |  |  |
Net proceeds from (purchases of) FHLB, Federal Reserve Bank, and other restricted stock |  | 12,846,000 | 5,471,000 | $ 16,233,000 |  |
Shareholders' equity |  | 1,117,086,000 | 1,052,795,000 | 956,816,000 |  | $ 920,964,000
Accrued interest receivable |  | $ 80,412,000 | 38,072,000 |  |  |
Contractual payment of principal duration (days) |  | 90 days |  |  |  |
Contractual terms for loans (months) |  | 6 months |  |  |  |
Maximum loan to value ratio |  | 80.00% |  |  |  |
Threshold period past due, writeoff |  | 120 days |  |  |  |
Goodwill and other intangibles |  | $ 14,298,000 | 15,195,000 | 16,499,000 |  |
FHLB, Federal Reserve Bank, and other restricted stock |  | 71,368,000 | 84,214,000 |  |  |
Federal Home Loan Bank stock |  | $ 46,100,000 | 60,800,000 |  |  |
Percentage of tax position that will be realized or sustained upon examination |  | 50.00% |  |  |  |
Number of share-based compensation plans | CompensationPlan |  | 4 |  |  |  |
Number of reportable segments (segment) | Segment |  | 2 |  |  |  |
Other |  | $ 8,767,000 | 11,380,000 | 10,723,000 |  |
Net deferred tax (asset) liability |  |  | 11,740,000 |  |  |
Collaborative Arrangement |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Other |  | 12,600,000 | 7,700,000 | 8,400,000 |  |
Interest-only GNMA securities |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Interest-only GNMA securities |  |  | $ 16,272,000 |  |  |
Net proceeds from (purchases of) FHLB, Federal Reserve Bank, and other restricted stock |  | 15,400,000 |  |  |  |
Other investments, realized gain |  | $ 1,000,000.0 |  |  |  |
Accounting Standards Update 2016-13 |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Initial allowance on loans purchased with credit deterioration | $ 200,000 |  |  |  |  |
Financing receivable, purchased with credit deterioration, discount | 300,000 |  |  |  |  |
Interest rate swaps | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Number of interest rate swaps | Swap |  | 155 | 140 |  |  |
Derivative notional amount |  | $ 1,400,000,000 | $ 1,400,000,000 |  |  |
Interest rate swaps | Cash Flow Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Number of interest rate swaps | contract |  | 5 | 4 |  |  |
Derivative notional amount |  | $ 1,100,000,000 | $ 725,000,000.0 |  |  |
Interest rate swaps | Fair Value Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Number of interest rate swaps | position |  | 24 |  |  |  |
Derivative notional amount |  | $ 272,300,000 |  |  |  |
Credit contracts | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative notional amount |  | $ 177,200,000 | 167,100,000 |  |  |
ESPP |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Purchase price under employee stock purchase plan (percent) |  | 85.00% |  |  | 85.00% |
Purchase price under employee stock purchase plan, discount (percent) |  | 15.00% |  |  | 15.00% |
Retained Earnings |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity |  | $ 438,581,000 | 381,519,000 | 316,651,000 |  | 258,076,000
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity |  | $ (5,764,000) | (1,250,000) | $ (22,663,000) |  | (359,000)
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity |  |  | (61,475,000) |  |  |
Net deferred tax (asset) liability | (21,510,000) |  |  |  |  |
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity | $ (61,500,000) |  | $ (61,475,000) |  |  |
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings | Accounting Standards Update 2016-01 |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity |  |  |  |  |  | 1,041,000
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01 |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Shareholders' equity |  |  |  |  |  | $ (1,041,000)

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