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Revenue Recognition - Change in Contract Liabilities (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Revenue from Contract with Customer [Abstract]    
Balance as of January 1 $ 215.8 $ 195.2
Revenue recognized (31.2) (32.2)
Revenue deferred 39.6 45.5
Acquired liabilities 0.2 0.0
Balance as of March 31 $ 224.4 $ 208.5