Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 229,583	$ 820,209
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(694)	(24,588)
Estimated Fair Value, 12 Months or More	523,463	110,947
Gross Unrealized Losses 12 Months or More	(23,726)	(23,078)
Total Estimated Fair Value	753,046	931,156
Total Gross Unrealized Losses	(24,420)	(47,666)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	0	2,401
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	0	(237)
Estimated Fair Value, 12 Months or More	24,356	21,122
Gross Unrealized Losses 12 Months or More	(13,236)	(18,280)
Total Estimated Fair Value	24,356	23,523
Total Gross Unrealized Losses	(13,236)	(18,517)
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	2,318	2,035
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(3)	(59)
Estimated Fair Value, 12 Months or More	0	0
Gross Unrealized Losses 12 Months or More	0	0	[1]
Total Estimated Fair Value	2,318	2,035
Total Gross Unrealized Losses	(3)	(59)
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	111,646	632,231
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(419)	(22,844)
Estimated Fair Value, 12 Months or More	368,706	65,324
Gross Unrealized Losses 12 Months or More	(9,411)	(4,319)	[1]
Total Estimated Fair Value	480,352	697,555
Total Gross Unrealized Losses	(9,830)	(27,163)
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	112,473	183,542
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(229)	(1,448)
Estimated Fair Value, 12 Months or More	130,401	24,501
Gross Unrealized Losses 12 Months or More	(1,079)	(479)
Total Estimated Fair Value	242,874	208,043
Total Gross Unrealized Losses	(1,308)	(1,927)
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	3,146
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(43)
Estimated Fair Value, 12 Months or More	0
Gross Unrealized Losses 12 Months or More	0
Total Estimated Fair Value	3,146
Total Gross Unrealized Losses	$ (43)

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