Mortgage-Backed Securities Available for Sale - Estimated Market Value and Unrealized Loss for Mortgage-Backed Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Less than 12 months, Estimated Market Value	$ 68,594	$ 17,182
Less than 12 months, Unrealized Losses	(518)	(204)
12 months or longer, Estimated Market Value	45,745	43,470
12 months or longer, Unrealized Losses	(10,440)	(11,530)
Total Estimated Market Value	114,339	60,652
Total Unrealized Losses	(10,958)	(11,734)
Mortgage-Backed Securities [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Less than 12 months, Estimated Market Value	204,095	21,057
Less than 12 months, Unrealized Losses	(4,527)	(64)
12 months or longer, Estimated Market Value	3,222
12 months or longer, Unrealized Losses	(2)
Total Estimated Market Value	207,317	21,057
Total Unrealized Losses	(4,529)	(64)
Mortgage-Backed Securities [Member] | FHLMC [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Less than 12 months, Estimated Market Value	125,115	16,186
Less than 12 months, Unrealized Losses	(2,352)	(53)
12 months or longer, Estimated Market Value	3,222
12 months or longer, Unrealized Losses	(2)
Total Estimated Market Value	128,337	16,186
Total Unrealized Losses	(2,354)	(53)
Mortgage-Backed Securities [Member] | FNMA [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Less than 12 months, Estimated Market Value	78,980	4,871
Less than 12 months, Unrealized Losses	(2,175)	(11)
Total Estimated Market Value	78,980	4,871
Total Unrealized Losses	$ (2,175)	$ (11)