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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
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Schedule Of Unrealized Losses On Securities [Line Items] |  |
Estimated Fair Value, Less Than 12 Months | $ 773,932 | $ 493,938
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (13,175) | (3,881)
Estimated Fair Value, 12 Months or More | 85,152 | 293,076
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (9,326) | (14,910)
Total Estimated Fair Value | 859,084 | 787,014
Total Gross Unrealized Losses | (22,501) | (18,791)
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 | 28,952 | 29,957
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (7,124) | (7,497)
Total Estimated Fair Value | 28,952 | 29,957
Total Gross Unrealized Losses | (7,124) | (7,497)
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 | 4,898 | 6,798
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (11) | (20)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 4,898 | 6,798
Total Gross Unrealized Losses | (11) | (20)
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 | 33,883 |
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (561) |
Estimated Fair Value, 12 Months or More | 0 |
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 |
Total Estimated Fair Value | 33,883 |
Total Gross Unrealized Losses | (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 | 670,708 | 436,011
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (11,630) | (3,293)
Estimated Fair Value, 12 Months or More | 56,200 | 263,119
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | (2,202) | (7,413)
Total Estimated Fair Value | 726,908 | 699,130
Total Gross Unrealized Losses | (13,832) | (10,706)
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 | 14,534 |
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (142) |
Estimated Fair Value, 12 Months or More | 0 |
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 |
Total Estimated Fair Value | 14,534 |
Total Gross Unrealized Losses | (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 | 16,632 | 12,316
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (69) | (85)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 16,632 | 12,316
Total Gross Unrealized Losses | (69) | (85)
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 | 33,277 | 7,208
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses | (762) | (58)
Estimated Fair Value, 12 Months or More | 0 | 0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 0 | 0
Total Estimated Fair Value | 33,277 | 7,208
Total Gross Unrealized Losses | $ (762) | $ (58)

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