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Debt - Debt Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Oct. 01, 2018
Sep. 30, 2018
Debt Instrument [Line Items]            
Interest paid $ 7.8 $ 6.7 $ 3.3 $ 1.1    
Revolving Credit Facility and Senior Notes            
Debt Instrument [Line Items]            
Debt instrument, interest rate 7.50%          
Wells Fargo Revolving Credit Facility            
Debt Instrument [Line Items]            
Maturity period prior to the final maturity of unsecured senior notes 60 days          
Subsequent Event | Revolving Credit Facility and Senior Notes            
Debt Instrument [Line Items]            
Debt instrument, interest rate         8.50% 6.50%
Subsequent Event | Wells Fargo Revolving Credit Facility            
Debt Instrument [Line Items]            
Debt instrument, interest rate           1.00%