Derivative Activities - Derivative Fair Value Income (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2013
Derivatives Fair Value [Line Items]
Derivatives that do not qualify or are not designated for hedge accounting, related to discontinuance of hedge accounting	$ 103.8	$ 22.4

Gain included in earnings for the period from the change in fair value of derivative instruments not 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 Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(1)(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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration