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3. Fair Value Measurements Fair Value and Gross Unrealized Losses with Unrealized Losses (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Price Change Debt Security 5.00%  
Tolerable variance Level 2 debt security pricing 1.00%  
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value $ 43.9 $ 11.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 168.7 160.5
Gross unrealized losses for investments in a loss position 12 months or more 0.7 0.3
Gross unrealized losses for investments in a loss position less than 12 months $ 0.8 $ 1.5
Cost Method Investment, Percentage Owned 35.00%