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Business Acquisitions - Schedule of Pro Forma Information Combines Consolidated Results of Company and Business Acquisitions (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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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
Business Acquisition Pro Forma Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net patient service revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,015,167 |  | $ 1,950,972 |  |
Net income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 260,485 | [1] | $ 255,183 | [1] |
Net income per share, Basic |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.35 | [2] | $ 5.35 | [2] |
Net income per share, Diluted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.24 | [2] | $ 5.23 | [2] |
Weighted average shares, Basic |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 48,693 | [2] | 47,706 | [2] |
Weighted average shares, Diluted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49,691 | [2] | 48,796 | [2] |
Effective tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 37.94% | [1] | 38.29% | [1] | 34.86%
Effective tax rate, prior year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 38.29% |  |
Basic weighted average shares outstanding | 49,113 | [3] | 48,938 | [3] | 48,543 | [3] | 48,269 | [3] | 48,126 | [3] | 47,990 | [3] | 47,531 | [3] | 47,149 | [3] | 48,693 |  | 47,706 |  | 46,630
Diluted weighted average shares outstanding | 50,106 | [3] | 49,809 | [3] | 49,545 | [3] | 49,399 | [3] | 49,132 | [3] | 48,935 | [3] | 48,730 | [3] | 48,361 | [3] | 49,691 |  | 48,796 |  | 47,570
[1] | The comparison of net income is affected by the change in the effective tax rate. The effective tax rate was 37.94% for the year ended December 31, 2012 as compared to 38.29% for the year ended December 31, 2011.
[2] | The comparison of net income per share is affected by the changes in the number of weighted average shares outstanding in each period. The basic and diluted weighted average shares outstanding for the year ended December 31, 2012 were 48.7 million and 49.7 million, respectively, as compared to 47.7 million and 48.8 million, respectively, for the year ended December 31, 2011.
[3] | Basic and diluted per share amounts are computed for each of the periods presented. Accordingly, the sum of the quarterly per share amounts may not agree with the full year amount.

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