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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | $ 493,938 | $ 229,583
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (3,881) | (694)
Estimated Fair Value, 12 Months or More | 293,076 | 523,463
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (14,910) | (23,726)
Total Estimated Fair Value | 787,014 | 753,046
Total Gross Unrealized Losses | (18,791) | (24,420)
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 | 29,957 | 24,356
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (7,497) | (13,236)
Total Estimated Fair Value | 29,957 | 24,356
Total Gross Unrealized Losses | (7,497) | (13,236)
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 | 6,798 | 2,318
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (20) | (3)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 6,798 | 2,318
Total Gross Unrealized Losses | (20) | (3)
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 | 16,596 |
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (247) |
Estimated Fair Value, 12 Months or More | 0 |
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 |
Total Estimated Fair Value | 16,596 |
Total Gross Unrealized Losses | (247) |
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 | 436,011 | 111,646
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (3,293) | (419)
Estimated Fair Value, 12 Months or More | 263,119 | 368,706
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (7,413) | (9,411)
Total Estimated Fair Value | 699,130 | 480,352
Total Gross Unrealized Losses | (10,706) | (9,830)
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 | 15,009 |
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (178) |
Estimated Fair Value, 12 Months or More | 0 |
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 |
Total Estimated Fair Value | 15,009 |
Total Gross Unrealized Losses | (178) |
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 | 12,316 | 112,473
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (85) | (229)
Estimated Fair Value, 12 Months or More | 0 | 130,401
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | (1,079)
Total Estimated Fair Value | 12,316 | 242,874
Total Gross Unrealized Losses | (85) | (1,308)
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 | 7,208 | 3,146
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (58) | (43)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 7,208 | 3,146
Total Gross Unrealized Losses | $ (58) | $ (43)

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