Acquisitions (Narrative) (Details) (USD $)	12 Months Ended								0 Months Ended
	Dec. 31, 2011 Centers	Dec. 31, 2010 Centers	Dec. 31, 2009	Dec. 31, 2011 Unfavorable Lease Liability	Dec. 31, 2011 Contingent Purchase Price Payable [Member]	Dec. 31, 2011 Noncontrolling Interest Centers	Dec. 31, 2011 Controlling Interest [Member] Centers	Dec. 31, 2010 Controlling Interest [Member] Centers	Sep. 01, 2011 Acquired Nsc Centers [Member] Centers	Dec. 31, 2011 Acquired Nsc Centers [Member]
Business Acquisition [Line Items]
Number of business acquisitions		0				2	24	7
Number of business units of acquiree in single transaction									17
Number of business units of acquiree in separate transactions	9
Goodwill deductible for tax purposes	$ 212,576,000	$ 55,400,000								$ 110,000,000
Amount paid for acquisitions									135,000,000
Purchase Price Payable	5,236,000	3,895,000
Payments to Acquire Businesses, Net of Cash Acquired	239,223,000	53,690,000	95,826,000
Business Acquisition Purchase Price Allocation Withheld Contingent Upon Exercise									1,700,000
Purchase Price Placed in Escrow									3,500,000
Business Acquisition Purchase Price Allocation Maximum Amount for Working Capital Adjustments									500,000
Other operating expenses in acquisition related costs, primarily attorney fees	3,783,000	248,000
Post Acquisition Adjustments - Liabilities	3,100,000			1,930,000	3,100,000
Post Acquisition Adjustment - Property and Equipment	4,900,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High										$ 7,500,000