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Summary of Significant Accounting Policies - Textual (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
state
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Finite-Lived Intangible Assets [Line Items]      
Number of states in which Entity operates | state 36    
Collection success rate of denied payments (percent) 70.00%    
Minimum accrual of loss contingency needed to accrue for related legal fees $ 5.0    
Advertising expense $ 6.3 $ 7.5 $ 7.3
Payor Source [Member] | Sales Revenue, Net [Member]      
Finite-Lived Intangible Assets [Line Items]      
Percentage of Medicare patient discharges (percent) 100.00% 100.00% 100.00%
Payor Source [Member] | Sales Revenue, Net [Member] | Medicare [Member] | RAC Audits [Member]      
Finite-Lived Intangible Assets [Line Items]      
Percentage of Medicare patient discharges (percent) 1.00%    
Accounting Standards Update 2016-09 [Member]      
Finite-Lived Intangible Assets [Line Items]      
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ 8.8    
Excess Tax Benefit from Share-based Compensation, Operating Activities   $ 17.3  
Payments Related to Tax Withholding for Share-based Compensation   $ 11.6 $ 17.2