Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) (USD $)	Jun. 30, 2013		Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 744,297,000		$ 135,599,000
Gross Unrealized Losses Less Than 12 Months	(20,700,000)		(464,000)
Estimated Fair Value, 12 Months or More	21,706,000		23,350,000
Gross Unrealized Losses 12 Months or More	(24,061,000)		(28,496,000)
Total Estimated Fair Value	766,003,000		158,949,000
Total Gross Unrealized Losses	44,761,000		28,960,000
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	4,798,000
Gross Unrealized Losses Less Than 12 Months	(26,000)
Estimated Fair Value, 12 Months or More	0
Gross Unrealized Losses 12 Months or More	0
Total Estimated Fair Value	4,798,000
Total Gross Unrealized Losses	26,000
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	33,000		0
Gross Unrealized Losses Less Than 12 Months	(23,000)		0
Estimated Fair Value, 12 Months or More	21,706,000		23,316,000
Gross Unrealized Losses 12 Months or More	(24,061,000)		(28,496,000)
Total Estimated Fair Value	21,739,000		23,316,000
Total Gross Unrealized Losses	24,084,000		28,496,000
Obligations of U.S. Government Agencies [Member] | Mortgage Backed Securities - Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	2,096,000		0
Gross Unrealized Losses Less Than 12 Months	(27,000)		0
Estimated Fair Value, 12 Months or More	0		13,000
Gross Unrealized Losses 12 Months or More	0	[1]	0	[2]
Total Estimated Fair Value	2,096,000		13,000
Total Gross Unrealized Losses	27,000		0
US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities - Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	550,385,000		76,296,000
Gross Unrealized Losses Less Than 12 Months	(17,643,000)		(392,000)
Estimated Fair Value, 12 Months or More	0		21,000
Gross Unrealized Losses 12 Months or More	0	[1]	0	[2]
Total Estimated Fair Value	550,385,000		76,317,000
Total Gross Unrealized Losses	17,643,000		392,000
US Government-sponsored Enterprises Debt Securities [Member] | Other Government Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value, Less Than 12 Months	186,985,000		59,303,000
Gross Unrealized Losses Less Than 12 Months	(2,981,000)		(72,000)
Estimated Fair Value, 12 Months or More	0		0
Gross Unrealized Losses 12 Months or More	0		0
Total Estimated Fair Value	186,985,000		59,303,000
Total Gross Unrealized Losses	$ 2,981,000		$ 72,000

[1]

[2] Gross unrealized losses related to these types of securities are less than $1 thousand.