Cash Equivalents and Investments - Maturities of Fixed Income Investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Schedule of Fixed Income Securities Maturities [Abstract]
Amortized cost due within one year	$ 431.4
Gross unrealized gains due within one year	0.5
Gross unrealized losses due within one year	0
Estimated fair value due within one year	431.9
Amortized cost due between one and five years	1,188.2
Gross unrealized gains due between one and five years	1.4
Gross unrealized losses due between one and five year	(1.0)
Estimated fair value due between one and five year	1,188.6
Total investments, amortized cost	1,619.6
Gross Unrealized Gains	1.9
Gross Unrealized Losses	(1.0)
Total investments, estimated fair value	$ 1,620.5