Selected Quarterly Financial Information - Summary of Company's Selected Quarterly Financial Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Net patient service revenue	$ 567,387	$ 554,730	$ 529,180	$ 502,715	$ 471,332	$ 473,134	$ 449,530	$ 422,616	$ 2,154,012	$ 1,816,612	$ 1,588,248
Practice salaries and benefits	353,316	346,879	331,922	329,201	290,671	292,030	275,951	272,261	1,361,318	1,130,913	970,396
Practice supplies and other operating expenses	24,027	19,445	19,416	19,500	19,276	17,606	17,956	16,985	82,388	71,823	66,815
General and administrative expenses	55,636	54,654	54,601	53,318	50,271	48,200	48,200	46,869	218,209	193,540	170,356
Depreciation and amortization	10,491	10,461	9,870	9,144	8,091	7,925	7,687	7,113	39,966	30,816	25,292
Total operating expenses	443,470	431,439	415,809	411,163	368,309	365,761	349,794	343,228	1,701,881	1,427,092	1,232,859
Income from operations	123,917	123,291	113,371	91,552	103,023	107,373	99,736	79,388	452,131	389,520	355,389
Investment income	526	372	396	402	681	422	365	428	1,696	1,896	1,495
Interest expense	(1,046)	(1,507)	(1,673)	(1,189)	(1,219)	(624)	(848)	(554)	(5,415)	(3,245)	(3,639)
Income before income taxes	123,397	122,156	112,094	90,765	102,485	107,171	99,253	79,262	448,412	388,171	353,245
Income tax provision	44,423	45,198	42,876	35,398	36,382	41,261	38,709	30,912	167,895	147,264	135,248
Net income	$ 78,974	$ 76,958	$ 69,218	$ 55,367	$ 66,103	$ 65,910	$ 60,544	$ 48,350	$ 280,517	$ 240,907	$ 217,997
Net income, Basic	$ 0.79	$ 0.77	$ 0.70	$ 0.56	$ 0.67	$ 0.67	$ 0.62	$ 0.50	$ 2.83	$ 2.47	$ 2.28
Net income, Diluted	$ 0.78	$ 0.76	$ 0.68	$ 0.55	$ 0.66	$ 0.66	$ 0.61	$ 0.49	$ 2.78	$ 2.42	$ 2.23
Weighted average shares, Basic	99,469	99,506	99,116	98,618	98,226	97,876	97,086	96,538	99,112	97,386	95,412
Weighted average shares, Diluted	101,080	101,178	101,098	100,784	100,212	99,618	99,090	98,798	100,969	99,382	97,592