Acquisitions (Narrative) (Details) (USD $)	12 Months Ended						0 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2012 Centers	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Noncontrolling Interest Number Of Centers [Member] Centers	Dec. 31, 2012 Controlling Interest [Member] Centers	Dec. 31, 2011 Controlling Interest [Member] Centers	Sep. 01, 2011 Acquired Nsc Centers [Member] Centers	Dec. 31, 2012 Acquired Nsc Centers [Member]	Dec. 31, 2011 Acquired Nsc Centers [Member]	Dec. 31, 2012 Prior Year Acquisitions [Member] Working Capital Adjustment Settlement Member [Member]
Business Acquisition [Line Items]
Number of business acquisitions				2	17	24
Number of business units of acquiree in single transaction							17
Number of business units of acquiree in separate transactions
Goodwill deductible for tax purposes	$ 260,547,000	$ 212,576,000							$ 110,000,000
Purchase Price Payable		5,236,000
Payments to Acquire Businesses, Net of Cash Acquired	277,388,000	239,223,000	53,690,000							115,000
Business Acquisition Purchase Price Allocation Withheld Contingent Upon Exercise	2,744,000
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	700,000	3,783,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High							7,500,000
Commitments, Fair Value Disclosure	2,744,000	3,100,000
Escrow Release								$ 3,500,000