Investments - Held-to-Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Marketable investments classified as held-to-maturity:
Amortized Cost	$ 81.9	$ 160.0
Gross Unrealized Losses		(0.2)
Fair Value	81.9	159.8
Government-sponsored enterprises
Marketable investments classified as held-to-maturity:
Amortized Cost	44.2	53.3
Gross Unrealized Losses		(0.2)
Fair Value	44.2	53.1
Corporate notes and bonds
Marketable investments classified as held-to-maturity:
Amortized Cost	37.7	106.7
Fair Value	$ 37.7	$ 106.7