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Basis of Financial Statements - Description of the Business (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Net asset value $ 1,203.9 $ 1,199.7 $ 1,146.8 $ 1,153.1