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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | $ 431,247 | $ 773,932
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (3,386) | (13,175)
Estimated Fair Value, 12 Months or More | 456,539 | 85,152
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (15,148) | (9,326)
Total Estimated Fair Value | 887,786 | 859,084
Total Gross Unrealized Losses | (18,534) | (22,501)
Corporate Securities [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 3,993 |
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (4) |
Estimated Fair Value, 12 Months or More | 0 |
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 |
Total Estimated Fair Value | 3,993 |
Total Gross Unrealized Losses | (4) |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 0 | 0
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | 0 | 0
Estimated Fair Value, 12 Months or More | 19,120 | 28,952
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (4,379) | (7,124)
Total Estimated Fair Value | 19,120 | 28,952
Total Gross Unrealized Losses | (4,379) | (7,124)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Residential [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 5,584 | 4,898
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (21) | (11)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 5,584 | 4,898
Total Gross Unrealized Losses | (21) | (11)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Commercial [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 48,322 | 33,883
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (962) | (561)
Estimated Fair Value, 12 Months or More | 32,683 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (894) | 0
Total Estimated Fair Value | 81,005 | 33,883
Total Gross Unrealized Losses | (1,856) | (561)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 351,222 | 670,708
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (2,295) | (11,630)
Estimated Fair Value, 12 Months or More | 400,984 | 56,200
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (9,746) | (2,202)
Total Estimated Fair Value | 752,206 | 726,908
Total Gross Unrealized Losses | (12,041) | (13,832)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Commercial [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 13,985 | 14,534
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (71) | (142)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 13,985 | 14,534
Total Gross Unrealized Losses | (71) | (142)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 997 | 16,632
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (1) | (69)
Estimated Fair Value, 12 Months or More | 99 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 1,096 | 16,632
Total Gross Unrealized Losses | (1) | (69)
US States and Political Subdivisions Debt Securities [Member] | Other Government - Sponsored Enterprises [Member] |  |
Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | 7,144 | 33,277
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (32) | (762)
Estimated Fair Value, 12 Months or More | 3,653 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (129) | 0
Total Estimated Fair Value | 10,797 | 33,277
Total Gross Unrealized Losses | $ (161) | $ (762)

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