Mortgage-Backed Securities Available for Sale - Amortized Cost and Estimated Market Value of 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]
Gross Unrealized Losses	$ (10,958)	$ (11,734)
Estimated Market Value	226,753	213,593
Mortgage-Backed Securities [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Amortized Cost	391,077	323,414
Gross Unrealized Gains	6,027	10,507
Gross Unrealized Losses	(4,529)	(64)
Estimated Market Value	392,575	333,857
Mortgage-Backed Securities [Member] | FHLMC [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Amortized Cost	162,588	118,294
Gross Unrealized Gains	623	1,284
Gross Unrealized Losses	(2,354)	(53)
Estimated Market Value	160,857	119,525
Mortgage-Backed Securities [Member] | FNMA [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Amortized Cost	227,718	204,296
Gross Unrealized Gains	5,230	9,017
Gross Unrealized Losses	(2,175)	(11)
Estimated Market Value	230,773	213,302
Mortgage-Backed Securities [Member] | GNMA [Member]
Schedule Of Mortgage Securities And Similar Investments [Line Items]
Amortized Cost	771	824
Gross Unrealized Gains	174	206
Gross Unrealized Losses
Estimated Market Value	$ 945	$ 1,030