Investment Securities - Securities with Other-than-Temporary-Impairment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Total other-than-temporary-impairment recognized in earnings	$ 202	$ 0	$ 1,078
Amount of other-than-temporary-impairment recognized in earnings	202		1,078	1,414	1,409	3,927	24,795
Life-to-date		31,545
Non Agency Residential Mortgage Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Total other-than-temporary-impairment recognized in earnings	198		913
Amortized Cost	21,954	12,624	21,954
Amount of other-than-temporary-impairment recognized in earnings				938	521	2,997	4,429
Life-to-date		8,885
Non Agency Residential Mortgage Backed Securities [Member] | BAFC Ser 4 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2007	2007
Lowest Credit Rating	CCC	CCC	CCC
Total other-than-temporary-impairment recognized in earnings	139		299
Amortized Cost	12,319	9,969	12,319
Amount of other-than-temporary-impairment recognized in earnings				299		79	63
Life-to-date		441
Non Agency Residential Mortgage Backed Securities [Member] | CWALT Ser 73CB [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2005	2005
Lowest Credit Rating	D		D
Total other-than-temporary-impairment recognized in earnings	23		140
Amortized Cost	2,613		2,613
Amount of other-than-temporary-impairment recognized in earnings				151		207	83
Life-to-date		441
Non Agency Residential Mortgage Backed Securities [Member] | CWALT Ser 73CB [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2005	2005
Lowest Credit Rating	D		D
Total other-than-temporary-impairment recognized in earnings	21		21
Amortized Cost	3,762		3,762
Amount of other-than-temporary-impairment recognized in earnings				35		427	182
Life-to-date		644
Non Agency Residential Mortgage Backed Securities [Member] | HALO Ser 1R [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006	2006
Total other-than-temporary-impairment recognized in earnings			133
Amount of other-than-temporary-impairment recognized in earnings				133	16
Life-to-date		149
Non Agency Residential Mortgage Backed Securities [Member] | RAST A9 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2004	2004
Total other-than-temporary-impairment recognized in earnings			142
Amount of other-than-temporary-impairment recognized in earnings				142
Life-to-date		142
Non Agency Residential Mortgage Backed Securities [Member] | RFMSI Ser S10 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006	2006
Lowest Credit Rating	D	D	D
Total other-than-temporary-impairment recognized in earnings	15		178
Amortized Cost	3,260	2,655	3,260
Amount of other-than-temporary-impairment recognized in earnings				178	165	76	249
Life-to-date		668
Non Agency Residential Mortgage Backed Securities [Member] | CWHL 2006-10 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006
Amount of other-than-temporary-impairment recognized in earnings						309	762
Life-to-date		1,071
Non Agency Residential Mortgage Backed Securities [Member] | CWHL 2005-20 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2005
Amount of other-than-temporary-impairment recognized in earnings						39	72
Life-to-date		111
Non Agency Residential Mortgage Backed Securities [Member] | FHASI Ser 4 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2007
Amount of other-than-temporary-impairment recognized in earnings					340	629	223
Life-to-date		1,192
Non Agency Residential Mortgage Backed Securities [Member] | RFMSI Ser S9 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006
Amount of other-than-temporary-impairment recognized in earnings						923	1,880
Life-to-date		2,803
Non Agency Residential Mortgage Backed Securities [Member] | RALI QS2 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006
Amount of other-than-temporary-impairment recognized in earnings						278	739
Life-to-date		1,017
Non Agency Residential Mortgage Backed Securities [Member] | RFMSI Ser S10 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2006
Amount of other-than-temporary-impairment recognized in earnings						30	176
Life-to-date		206
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Total other-than-temporary-impairment recognized in earnings	4		165
Amortized Cost	4,012	8,750	4,012
Amount of other-than-temporary-impairment recognized in earnings				476	888	930	20,366
Life-to-date		22,660
Pooled Trust Preferred Securities [Member] | Reg Div Funding 2004 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2004	2004
Lowest Credit Rating	D	D	D
Total other-than-temporary-impairment recognized in earnings	4		165
Amortized Cost	4,012	4,012	4,012
Amount of other-than-temporary-impairment recognized in earnings				165		321	5,199
Life-to-date		5,685
Pooled Trust Preferred Securities [Member] | TROPC [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2003
Amount of other-than-temporary-impairment recognized in earnings					888	444	3,517
Life-to-date		4,849
Pooled Trust Preferred Securities [Member] | MM Community Funding IX [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2003
Lowest Credit Rating		D
Amortized Cost		2,067
Amount of other-than-temporary-impairment recognized in earnings						165	2,612
Life-to-date		2,777
Pooled Trust Preferred Securities [Member] | Pretsl XII [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2003
Lowest Credit Rating		C
Amortized Cost		2,671
Amount of other-than-temporary-impairment recognized in earnings							1,897
Life-to-date		1,897
Pooled Trust Preferred Securities [Member] | Pretsl XV [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2004
Amount of other-than-temporary-impairment recognized in earnings							3,374
Life-to-date		3,374
Pooled Trust Preferred Securities [Member] | Reg Div Funding 2005 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Debt Issuance Year		2005
Amount of other-than-temporary-impairment recognized in earnings				311			3,767
Life-to-date		$ 4,078