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Business Acquisitions - Additional Information (Detail)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2017
USD ($)
Company
shares
Dec. 31, 2016
USD ($)
Company
Acquisition
Business Acquisition [Line Items]      
Number of physician group practices acquired | Company   10 13
Number of neonatology practices acquired | Company   1 1
Number of maternal-fetal medicine practices acquired | Company   2 1
Number of other pediatric subspecialty practices acquired | Company   2 2
Number of pediatric multi-specialty practice acquired | Company   1  
Number of radiology practices acquired | Company   4  
Total business acquisition consideration, net of cash acquired   $ 554,000 $ 759,600
Cash payments to acquire businesses   531,700 756,100
Payments to acquire business by issue of common stock   $ 2,700  
Number of shares of common stock issued | shares   69,014  
Contingent consideration liability   $ 18,600 $ 3,500
Purchase consideration liability   $ 1,000  
Contractual restriction period on disposition or assignment of common stock   3 years  
Assets held for sale, current   $ 23,500  
Assets held for sale, other noncurrent assets   $ 22,500  
Business combination contingent consideration arrangements description   The contingent consideration of $18.6 million recorded during the year ended December 31, 2017 is related to an agreement to pay an additional cash amount based on the achievement of certain performance measures for the period through June 1, 2019.  
Restricted cash   $ 20,000  
Contingent consideration payments related to prior-period acquisitions   6,200  
Number of business acquired | Acquisition     15
Cash acquired from acquisition     $ 15,000
Number of third-party receivables companies acquired | Company     1
Number of anesthesiology practices acquired | Company     8
Number of pediatric cardiology practices acquired | Company     1
Number of patient engagement software company acquired | Company     1
Sale of minority interest percentage 75.00%    
Deconsolidation percentage 100.00%    
Prior-Period Acquisitions [Member]      
Business Acquisition [Line Items]      
Increase in deferred tax liabilities   500  
Increase in goodwill resulting from finalization of tax acquisition accounting   500  
Contingent Consideration [Member]      
Business Acquisition [Line Items]      
Range of the undiscounted amount under contingent consideration agreement, low   0  
Range of the undiscounted amount under contingent consideration agreement, high   20,000  
Restricted cash   $ 20,000  
Minimum [Member] | Contingent Consideration [Member]      
Business Acquisition [Line Items]      
Assumed discount rates for acquisition-date fair value   4.00%  
Maximum [Member] | Contingent Consideration [Member]      
Business Acquisition [Line Items]      
Assumed discount rates for acquisition-date fair value   4.75%