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Investments and Fair Value Measurements (Cost and Fair Value of Investments Classified by Maturity) (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Available-for-sale Securities, Debt Maturities [Abstract]    
Available-for-sale securities, Equity securities, exchange-traded funds, and mutual funds, Cost $ 27.5 $ 18.7
Available-for-sale securities, Equity securities, exchange-traded funds, and mutual funds, Fair Value 26.6 17.3
Available-for-sale securities, Cost 27.5 18.7
Available-for-sale securities, Fair Value 26.6 17.3
Held-to-maturity Securities, Debt Maturities [Abstract]    
Held-to-maturity securities, Due in one year or less, Cost 14.0 15.3
Held-to-maturity securities, Due within one year or less, Fair Value 14.0 15.3
Held-to-maturity securities, Due in one to three years, Cost 1.8 0.0
Held-to-maturity securities, Due in one to three years, Fair Value 1.8 0.0
Held-to-maturity securities, Cost 15.8 15.3
Held-to-maturity securities, Fair Value $ 15.8 $ 15.3