Cash Equivalents and Investments - Maturities of Fixed Income Investments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Fixed Income Securities Maturities [Abstract]
Amortized Cost	$ 1,143.9	$ 2,386.3
Total investments, estimated fair value	1,173.9	2,418.8
Fixed Income Securities
Schedule of Fixed Income Securities Maturities [Abstract]
Amortized cost due within one year	605.1
Amortized cost due between one and five years	522.7
Amortized Cost	1,127.8	2,370.4
Estimated fair value due within one year	603.9
Estimated fair value due between one and five year	516.5
Total investments, estimated fair value	$ 1,120.4	$ 2,365.5