Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Net revenue:
Managed care and other	$ 742,950	$ 711,092	$ 716,998	$ 686,059	$ 652,804	$ 625,079	$ 641,742	$ 632,366	$ 2,857,099		$ 2,551,991		$ 2,734,406
Dispensing	87,324	87,345	88,475	87,154	68,660	61,764	56,596	60,389	350,298		247,409		234,834
Total net revenue	830,274	798,437	805,473	773,213	721,464	686,843	698,338	692,755	3,207,397		2,799,400		2,969,240
Costs and expenses:
Cost of care	528,529	516,238	521,830	505,293	461,527	448,051	441,446	433,700	2,071,890		1,784,724		1,907,985
Cost of goods sold	82,859	81,662	82,855	81,038	64,479	57,636	53,404	56,519	328,414		232,038		218,630
Direct service costs and other operating expenses	145,016	135,574	140,333	136,589	136,250	130,038	131,779	131,567	557,512	[1]	529,634	[1]	566,582	[1]
Depreciation and amortization	15,316	15,239	15,152	14,781	15,335	15,069	14,267	13,952	60,488		58,623		54,682
Interest expense	534	537	576	600	1,080	457	494	471	2,247		2,502		2,233
Interest income	(400)	(350)	(857)	(412)	(516)	(592)	(858)	(815)	(2,019)		(2,781)		(3,275)
Total cost and expenses	771,854	748,900	759,889	737,889	678,155	650,659	640,532	635,394	3,018,532		2,604,740		2,746,837
Income before income taxes	58,420	49,537	45,584	35,324	43,309	36,184	57,806	57,361	188,865		194,660		222,403
Provision (benefit) for income taxes	21,418	(16,725)	18,611	14,534	13,570	4,829	23,575	23,063	37,838		65,037		83,744
Net income	37,002	66,262	26,973	20,790	29,739	31,355	34,231	34,298	151,027		129,623		138,659
Weighted average number of common shares outstanding-basic	27,505	27,521	27,317	27,199	27,724	29,900	31,301	33,051	27,386		30,478		33,779
Weighted average number of common shares outstanding-diluted	28,020	28,042	27,717	27,747	28,300	30,438	31,903	33,656	27,882		31,058		34,441
Net income per common share-basic (in dollars per share)	$ 1.35	$ 2.41	$ 0.99	$ 0.76	$ 1.07	$ 1.05	$ 1.09	$ 1.04	$ 5.51		$ 4.25		$ 4.10
Net income per common share-diluted (in dollars per share)	$ 1.32	$ 2.36	$ 0.97	$ 0.75	$ 1.05	$ 1.03	$ 1.07	$ 1.02	$ 5.42		$ 4.17		$ 4.03
Stock compensation expense	$ 3,848	$ 4,468	$ 4,365	$ 5,102	$ 4,010	$ 4,425	$ 4,205	$ 4,778	$ 17,783		$ 17,418		$ 15,102

[1] Includes stock compensation expense of $15,102, $17,418 and $17,783 for the years ended December 31, 2010, 2011 and 2012, respectively.