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Commitments and Contingencies, Liquidity Option Agreement (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Liquidity Option Agreement [Abstract]          
Change in fair market value of Liquidity Option Agreement $ 26.6 $ 8.9 $ 84.4 $ 16.4  
Percentage of capital stock we agreed to purchase under liquidity option agreement     100.00%    
Liquidity Option exercise period     90 days    
Number of units held by limited partner 54,807,352   54,807,352    
Valuation, Income Approach [Member] | Liquidity Option Agreement [Member]          
Liquidity Option Agreement [Abstract]          
Derivative Liability $ 474.4   $ 474.4   $ 390.0