Investments - Gross Unrealized Losses (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Continuous unrealized loss position, Fair Value
Fair Value, Total	$ 7.0	$ 111.9
Continuous unrealized loss position, Gross Unrealized Loss
Gross Unrealized Loss, Total		(0.2)
Government-sponsored enterprises
Continuous unrealized loss position, Fair Value
Less than one year, Fair value		48.1
Fair Value, Total		48.1
Continuous unrealized loss position, Gross Unrealized Loss
Less than one year, Gross Unrealized Loss		(0.2)
Gross Unrealized Loss, Total		(0.2)
Corporate notes and bonds
Continuous unrealized loss position, Fair Value
Less than one year, Fair value	7.0	63.8
Fair Value, Total	$ 7.0	$ 63.8