Derivative Financial Instruments - Additional Information (Detail) (Forward Sales Commitments [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Forward Sales Commitments [Member]
Derivative [Line Items]
Derivative assets subject to master netting arrangements	$ 2.9	$ 0
Derivative liabilities subject to master netting arrangements	7.2	0
Net settlement liability subject to master repurchase arrangements	$ 4.8	$ 0

Fair value of the assets less the liabilities of a derivative or group of derivatives.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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-Section 50

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

-Publisher FASB

-Name Accounting Standards Codification

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

-Publisher FASB

-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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