Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2012 Participant ClassAction
Contingencies [Line Items]
Number of putative class actions	5
Payment of litigation expense	$ 55,000,000
Percentage of cash settlement of litigation expense	80.00%
Percentage of freely-tradable shares in settlement of litigation expense	20.00%
Percentage of exchange value stated in final prospectus	10.00%
Gross proceeds to be raised in the IPO	$ 600,000,000
Participants in ESBA filed an objection to preliminary approval	6
Final approval hearing date of the settlement	May 2, 2013

The cash inflow associated with the amount received from entity's first offering of stock to the public.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 19

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3255-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration