Fair Value of Financial Instruments - Gross Components of Purchases, Sales, and Settlements, Net, of Level 3 Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net Purchases, Sales & Settlements		$ (103,260)
Purchases	427,229	12,372
Sales	(27,263)	(64,627)
Settlements	(53,357)	(22,186)
Net Purchases, Sales & Settlements	346,609	(74,441)
Equity conversion feature of preferred stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases
Sales		(103,260)
Settlements
Net Purchases, Sales & Settlements		(103,260)
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	410,707	2,007
Settlements	(38,811)	(13,716)
Net Purchases, Sales & Settlements	371,896	(11,709)
Commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	4,999
Net Purchases, Sales & Settlements	4,999
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	1,326	10,365
Sales	(26,788)	(48,898)
Settlements	(14,224)	(6,696)
Net Purchases, Sales & Settlements	(39,686)	(45,229)
Municipals [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases	10,197
Settlements	(20)
Net Purchases, Sales & Settlements	10,177
Non-agency residential mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Sales	(475)	(15,729)
Settlements	(302)	(1,774)
Net Purchases, Sales & Settlements	$ (777)	$ (17,503)

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Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

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- Definition

Amount of purchases, sales, issues, and settlements (net) which have taken place in relation to assets measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Purchases that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Amount of purchases, sales, issues, and settlements in relation to liabilities measured at fair value on a recurring basis using unobservable inputs (level 3).

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Sales that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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