Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities Available-for-Sale and Trading Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 833,672	$ 886,031
Gross Unrealized Losses	(24,736)	(717)
Fair Value	813,708	907,498
Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	833,672	886,031
Gross Unrealized Gains	4,772	22,184
Gross Unrealized Losses	(24,736)	(717)
Fair Value	813,708	907,498
Federal National Mortgage Association ("FNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(10,187)	(243)
Federal National Mortgage Association ("FNMA") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		396,910
Gross Unrealized Gains		9,588
Gross Unrealized Losses		(243)
Fair Value		406,255
Collateralized Mortgage Obligation ("CMO") [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(4,200)	(301)
Collateralized Mortgage Obligation ("CMO") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	127,408	251,848
Gross Unrealized Gains	2,418	7,849
Gross Unrealized Losses	(4,200)	(301)
Fair Value	125,626	259,396
Collateralized Mortgage Obligation ("CMO") [Member] | Trading Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,590	12,590
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	12,590	12,590
Federal Home Loan Mortgage Corporation ("FHLMC") [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(3,251)	(117)
Federal Home Loan Mortgage Corporation ("FHLMC") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		58,596
Gross Unrealized Gains		1,171
Gross Unrealized Losses		(117)
Fair Value		59,650
Government National Mortgage Association ("GNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(2,727)	(54)
Government National Mortgage Association ("GNMA") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		129,288
Gross Unrealized Gains		3,221
Gross Unrealized Losses		(54)
Fair Value		132,455
Reverse Mortgages [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	(422)	(457)
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	(422)	(457)
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(4,368)
State and Political Subdivisions [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,058	3,120
Gross Unrealized Gains	153	89
Gross Unrealized Losses	(4,368)
Fair Value	55,843	3,209
U.S. Government and government sponsored enterprises ("GSE") [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(3)	(2)
U.S. Government and government sponsored enterprises ("GSE") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,688	46,726
Gross Unrealized Gains	148	266
Gross Unrealized Losses	(3)	(2)
Fair Value	43,833	46,990
Mortgage-backed Securities ("MBS") [Member] | Federal National Mortgage Association ("FNMA") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	380,368
Gross Unrealized Gains	475
Gross Unrealized Losses	(10,187)
Fair Value	370,656
Mortgage-backed Securities ("MBS") [Member] | Federal Home Loan Mortgage Corporation ("FHLMC") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,964
Gross Unrealized Gains	47
Gross Unrealized Losses	(3,251)
Fair Value	109,760
Mortgage-backed Securities ("MBS") [Member] | Government National Mortgage Association ("GNMA") [Member] | Available-For-Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,608
Gross Unrealized Gains	1,531
Gross Unrealized Losses	(2,727)
Fair Value	$ 108,412