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Long-Term Debt Long-Term Debt (Term Loan) (Details) - Term Loan [Member]
3 Months Ended
Mar. 31, 2016
USD ($)
Sep. 30, 2016
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]                    
Debt face amount $ 2,035,000,000.000               $ 1,243,000,000 $ 0
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 6.75    
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%