Basis of presentation (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended						6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Professional corporations Administrative services agreements	Jun. 30, 2012 Professional corporations Administrative services agreements	Jun. 30, 2013 Consolidated VIEs center item	Dec. 31, 2012 Consolidated VIEs	Jun. 30, 2013 Unconsolidated VIEs item	Dec. 31, 2012 Unconsolidated VIEs	Jun. 30, 2013 Unconsolidated VIEs Low end of the range	Dec. 31, 2012 Unconsolidated VIEs Low end of the range	Jun. 30, 2013 Unconsolidated VIEs High end of the range	Dec. 31, 2012 Unconsolidated VIEs High end of the range
Variable Interest Entities
Assets related to VIEs	$ 950,775,000		$ 950,775,000		$ 922,301,000			$ 74,000,000	$ 62,900,000
Number of variable interest entities								24		4
Number of operating centers of variable interest entities								44
Equity investments in joint ventures	781,000		781,000		575,000					800,000	600,000
Ownership interest (as a percent)												28.50%	28.50%	45.00%	45.00%
Net Patient Service Revenue and Allowances for Contractual Discounts
Net patient service revenue (as a percent)						18.90%	18.70%
Cost of revenues	$ 127,300,000	$ 122,000,000	$ 251,400,000	$ 242,400,000