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Fair Value Measurements - Summary of Changes in the Fair Values of level 3 Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Impairment $ (0.3)  
Level 3 | Corporate debt securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair Value, beginning of period 17.8 $ 0.0
Transfers from Level 2 0.0 21.4
Impairment (0.3) 0.0
Fair Value, end of period $ 17.5 $ 21.4