Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		$ 765,433,000	$ 135,599,000
Grous Unrealized Losses Less Than 12 Months		(24,191,000)	(464,000)
Estimated Fair Value, 12 Months or More		22,603,000	23,350,000
Gross Unrealized Losses 12 Months or More		(23,440,000)	28,496,000
Total Estimated Fair Value		788,036,000	158,949,000
Total Gross Unrealized Losses		(47,631,000)	28,960,000
Gross Unrealized Losses Related To Available For Sale Securities	Less than $1 thousand
US Government Agencies Residential Mortgage-backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		2,092,000	0
Grous Unrealized Losses Less Than 12 Months		(16,000)	0
Estimated Fair Value, 12 Months or More		0	13,000
Gross Unrealized Losses 12 Months or More		0	0
Total Estimated Fair Value		2,092,000	13,000
Total Gross Unrealized Losses		(16,000)	0
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		580,232,000	76,296,000
Grous Unrealized Losses Less Than 12 Months		(22,258,000)	(392,000)
Estimated Fair Value, 12 Months or More		0	21,000
Gross Unrealized Losses 12 Months or More		0	0
Total Estimated Fair Value		580,232,000	76,317,000
Total Gross Unrealized Losses		(22,258,000)	(392,000)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		183,075,000	59,303,000
Grous Unrealized Losses Less Than 12 Months		(1,894,000)	(72,000)
Estimated Fair Value, 12 Months or More		0	0
Gross Unrealized Losses 12 Months or More		0	0
Total Estimated Fair Value		183,075,000	59,303,000
Total Gross Unrealized Losses		(1,894,000)	(72,000)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		34,000	0
Grous Unrealized Losses Less Than 12 Months		(23,000)	0
Estimated Fair Value, 12 Months or More		22,603,000	23,316,000
Gross Unrealized Losses 12 Months or More		(23,440,000)	(28,496,000)
Total Estimated Fair Value		22,637,000	23,316,000
Total Gross Unrealized Losses		(23,463,000)	(28,496,000)
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		765,433,000
Grous Unrealized Losses Less Than 12 Months		(24,191,000)
Estimated Fair Value, 12 Months or More		22,603,000
Gross Unrealized Losses 12 Months or More		(23,440,000)
Total Estimated Fair Value		788,036,000
Total Gross Unrealized Losses		(47,631,000)
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months		0
Grous Unrealized Losses Less Than 12 Months		0
Estimated Fair Value, 12 Months or More		0
Gross Unrealized Losses 12 Months or More		0
Total Estimated Fair Value	0	0
Total Gross Unrealized Losses		0
Maximum [Member] | Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 1,000