Cash Equivalents and Investments - Maturities of Available for Sale Investments (Details) $ in Millions	Dec. 31, 2015 USD ($)
Cash Equivalents and Investments [Abstract]
Amortized cost due within one year	$ 504.1
Gross unrealized gains due within one year	0.1
Gross unrealized losses due within one year	(0.3)
Estimated fair value due within one year	503.9
Amortized cost due between one and five years	1,247.8
Gross unrealized gains due between one and five years	0.1
Gross unrealized losses due between one and five years	(3.6)
Estimated fair value due between one and five year	1,244.3
Total investments, amortized cost	1,751.9
Total investments, gross unrealized gains	0.2
Total investments, gross unrealized losses	(3.9)
Total investments, estimated fair value	$ 1,748.2