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Basis of Financial Statements - Description of the Business (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Jun. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]            
Net asset value $ 1,274.7 $ 1,232.2 $ 1,199.7 $ 1,170.1 $ 1,168.2 $ 1,153.1