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Long-Term Debt - Credit Facility (Details) - Revolving Credit Facility - Credit Facility - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Line of Credit Facility          
Letter of credit outstanding $ 15.2   $ 15.2    
Debt instrument, variable rate (percentage)     2.70%    
Debt instrument weighted average interest rate (percent) 5.30%   5.30%   5.40%
Commitment fee amount $ 0.6 $ 0.6 $ 1.1 $ 1.1  
Undrawn capacity under revolving credit facility $ 442.3   $ 442.3