Acquisitions and Divestitures - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Mar. 31, 2011 Facility	Sep. 30, 2011	Dec. 31, 2011	Oct. 03, 2011 Subsequent Event [Member]
Significant Acquisitions and Disposals [Line Items]
Number of shares of common stock valued at the closing date		432,407
Closing date price		$ 42.85
Total consideration net of cash acquired		$ 18,529,000
Number of shares issued to seller	60,855	372,992
Estimated weighted-average period intangible assets are to be amortized		20 years
Number of facilities acquired		10
Amortizing intangible assets recognized		6,419,000
Cash proceeds from divestiture			64,174,000
Gain recognized on divested assets			39,659,000
Goodwill included in book value of assets			10,300,000
Cash paid for acquiring facilities					10,000,000
Assets held for sale	$ 0		$ 26,883,000	$ 0