Equity - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Equity [Line Items]
Noncontrolling common units	18,023,435
Percentage of ownership interests classified as noncontrolling	32.00%
Noncash compensation expense	$ 0.7	$ 0.7
Unrecognized compensation expense	$ 3.6
Weighted average unvested shares outstanding	631,199	627,267

The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Name:	us-gaap_RestrictedStockExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration