Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Selected Quarterly Financial Data (Unaudited)
Net revenue	$ 721,464	$ 686,843	$ 698,338	$ 692,755	$ 749,210	$ 750,319	$ 741,658	$ 728,053	$ 2,799,400		$ 2,969,240		$ 2,641,814
Cost and expenses:
Cost of care	461,527	448,051	441,446	433,700	491,668	467,160	472,478	476,679	1,784,724		1,907,985		1,765,313
Cost of goods sold	64,479	57,636	53,404	56,519	52,492	55,071	54,771	56,296	232,038		218,630		203,336
Direct service costs and other operating expenses	136,250	130,038	131,779	131,567	147,130	141,581	139,617	138,254	529,634	[1]	566,582	[1]	465,710	[1]
Depreciation and amortization	15,335	15,069	14,267	13,952	13,075	13,950	14,235	13,422	58,623		54,682		47,268
Interest expense	1,080	457	494	471	482	482	584	685	2,502		2,233		2,424
Interest income	(516)	(592)	(858)	(815)	(809)	(846)	(803)	(817)	(2,781)		(3,275)		(6,245)
Total cost and expenses	678,155	650,659	640,532	635,394	704,038	677,398	680,882	684,519	2,604,740		2,746,837		2,477,806
Income before income taxes	43,309	36,184	57,806	57,361	45,172	72,921	60,776	43,534	194,660		222,403		164,008
Provision for income taxes	13,570	4,829	23,575	23,063	12,244	28,137	25,348	18,015	65,037		83,744		57,337
Net income	29,739	31,355	34,231	34,298	32,928	44,784	35,428	25,519	129,623		138,659		106,671
Weighted average number of common shares outstanding-basic	27,724	29,900	31,301	33,051	33,971	33,450	33,323	34,382	30,478		33,779		35,248
Weighted average number of common shares outstanding-diluted	28,300	30,438	31,903	33,656	34,730	34,171	33,800	35,074	31,058		34,441		35,416
Net income per common share-basic: (in dollars per share)	$ 1.07	$ 1.05	$ 1.09	$ 1.04	$ 0.97	$ 1.34	$ 1.06	$ 0.74	$ 4.25		$ 4.10		$ 3.03
Net income per common share-diluted: (in dollars per share)	$ 1.05	$ 1.03	$ 1.07	$ 1.02	$ 0.95	$ 1.31	$ 1.05	$ 0.73	$ 4.17		$ 4.03		$ 3.01
Stock compensation expense	$ 4,010	$ 4,425	$ 4,205	$ 4,778	$ 3,272	$ 3,596	$ 3,706	$ 4,528	$ 17,418		$ 15,102		$ 19,782

[1] Includes stock compensation expense of $19,782, $15,102, and $17,418 for the years ended December 31, 2009, 2010 and 2011, respectively.