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Long-Term Debt Long-Term Debt (Term Loan) (Details) - Term Loan [Member]
3 Months Ended
Mar. 31, 2016
USD ($)
Sep. 30, 2016
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]                  
Debt face amount $ 2,035,000,000.000             $ 1,243,000,000 $ 1,243,000,000
Leverage ratio (not more than)               6.75  
Leverage ratio, certain acquisitions (not more than) 6.0                
Leverage ratio, certain acquisitions threshold (in excess of) $ 50,000,000                
Scenario, Forecast [Member]                  
Debt Instrument [Line Items]                  
Leverage ratio (not more than)     5.5 5.75 6.0 6.25 6.5    
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 1.50% 2.75%              
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 3.00% 3.00%              
Base Rate [Member] | Minimum [Member]                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 0.50% 1.75%              
Base Rate [Member] | Maximum [Member]                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 2.00% 2.00%