Marketable Investments - Contractual Maturities of Available-for-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in less than one year	$ 168.1
Due in one to two years	80.8
Due in three to five years	62.9
Total	311.8
Fair Value
Due in less than one year	168.1
Due in one to two years	80.8
Due in three to five years	63.0
Marketable investments	$ 311.9	$ 0.0