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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Investment Securities [Line Items] |  |
Amortized Cost | $ 805,515 | $ 896,495
Gross Unrealized Gains | 25,708 | 8,906
Gross Unrealized Losses | 0 | (3,109)
Securities available for sale | 831,223 | 902,292
Estimated Fair Value, Less Than 12 Months | 3,755 | 166,470
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (10) | (637)
Estimated Fair Value, 12 Months or More | 0 | 235,380
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | (2,950)
Securities Continuous Unrealized Loss Position Fair Value | 3,755 | 401,850
SecuritiesContinuousUnrealizedLossPositionAggregateLosses | (10) | (3,587)
US States and Political Subdivisions Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 11,154 | 17,738
Gross Unrealized Gains | 243 | 171
Gross Unrealized Losses | 0 | 0
Securities available for sale | 11,397 | 17,909
Corporate Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 22,941 | 22,919
Gross Unrealized Gains | 1,444 | 1,043
Gross Unrealized Losses | 0 | 0
Securities available for sale | 24,385 | 23,962
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 481,109 | 660,777
Gross Unrealized Gains | 14,924 | 4,113
Gross Unrealized Losses | 0 | (2,943)
Securities available for sale | 496,033 | 661,947
Estimated Fair Value, Less Than 12 Months | 3,755 | 111,969
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (10) | (436)
Estimated Fair Value, 12 Months or More | 0 | 219,015
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | (2,801)
Securities Continuous Unrealized Loss Position Fair Value | 3,755 | 330,984
SecuritiesContinuousUnrealizedLossPositionAggregateLosses | (10) | (3,237)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 100,996 | 1,000
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | 0 | 0
Securities available for sale | 100,998 | 1,000
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Commercial [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 182,823 | 186,316
Gross Unrealized Gains | 8,357 | 2,983
Gross Unrealized Losses | 0 | (166)
Securities available for sale | 191,180 | 189,133
Estimated Fair Value, Less Than 12 Months |  | 54,501
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses |  | (201)
Estimated Fair Value, 12 Months or More |  | 16,365
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses |  | (149)
Securities Continuous Unrealized Loss Position Fair Value |  | 70,866
SecuritiesContinuousUnrealizedLossPositionAggregateLosses |  | (350)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Residential [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 6,492 | 7,745
Gross Unrealized Gains | 738 | 596
Gross Unrealized Losses | 0 | 0
Securities available for sale | $ 7,230 | $ 8,341

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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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