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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Fair Value Disclosures [Abstract]          
Net unrealized gain (loss) on securities $ (29.1) $ (37.5) $ 523.8 $ 133.8  
Securities owned not readily marketable 40.0   40.0   $ 59.2
Long-term debt fair value $ 1,891.0   $ 1,891.0   $ 1,958.0