Proforma Earnings Per Share (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Computation Of Earnings Per Share Line Items
Basic	$ 2.08	$ 1.56	$ 0.80
Diluted	$ 2.07	$ 1.54	$ 0.79

Pro forma basic earnings per share or earnings per unit, which is commonly presented in initial public offerings based on the terms of the offering.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph b

-Subparagraph 7

-Article 11

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration