Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
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)

Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($)
$ in Thousands

Dec. 31, 2015

Dec. 31, 2014

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