Cash Equivalents and Investments - Maturities of Fixed Income Investments (Details) $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Fixed Income Securities Maturities [Abstract]
Amortized cost due within one year	$ 1,092.0
Amortized cost due between one and five years	1,060.9
Total investments, amortized cost	2,152.9
Estimated fair value due within one year	1,091.7
Estimated fair value due between one and five year	1,059.3
Total investments, estimated fair value	$ 2,151.0