Derivative Financial Instruments - Additional Information (Detail) (Maximum, USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2012
Maximum
Derivative Financial Instruments [Line Items]
Increased in un-hedged exposure limit on mortgage loans	$ 115.0

The increase (decrease) during the period, which was recognized in earnings, in the unrealized gains or losses on derivative instruments that are not or are no longer designated as hedging instruments.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 20

-Section 35

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6439004&loc=d3e65527-113976

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 18

-Subparagraph a, b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Name:	us-gaap_IncreaseDecreaseInFairValueOfUnhedgedDerivativeInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration