Segment and geographic information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 item segment	Jun. 30, 2012	Dec. 31, 2012
Segment and geographic information
Number of operating lines of business			1
Number of operating segments			2
Total revenues	$ 178,109	$ 180,254	$ 352,086	$ 357,699
Facility gross profit	50,813	58,262	100,650	115,301
Depreciation and amortization	15,320	16,247	30,491	31,443
Total assets	950,775		950,775		922,301
Property and equipment	224,714		224,714		221,050
Capital expenditures			17,847		37,957
Acquisition-related goodwill and intangible assets	538,099		538,099		520,903
U. S Domestic
Segment and geographic information
Number of operating segments			8
Number of reporting segments			1
Total revenues	155,649	159,991	308,943	317,283
Facility gross profit	38,742	47,560	76,759	93,088
Depreciation and amortization	14,210	15,252	28,339	29,510
Total assets	809,266		809,266		780,691
Property and equipment	207,463		207,463		204,012
Capital expenditures			15,703		32,660
Acquisition-related goodwill and intangible assets	456,974		456,974		435,331
International
Segment and geographic information
Number of reporting segments			1
Total revenues	22,460	20,263	43,143	40,416
Facility gross profit	12,071	10,702	23,891	22,213
Depreciation and amortization	1,110	995	2,152	1,933
Total assets	141,509		141,509		141,610
Property and equipment	17,251		17,251		17,038
Capital expenditures			2,144		5,297
Acquisition-related goodwill and intangible assets	$ 81,125		$ 81,125		$ 85,572