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Significant Accounting Policies, Deferred Revenue (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Deferred Revenue [Abstract]        
Revenue recognized from amounts in beginning deferred revenue balance $ 30.3 $ 37.2 $ 103.5 $ 80.4