Cash, Cash Equivalents, and Investments - Maturities of Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale and Trading Investments [Abstract]
Amortized cost due within one year	$ 2,021.6
Gross unrealized gains due within one year	0.3
Gross unrealized losses due within one year	0
Estimated fair value due within one year	2,021.9
Amortized cost due between one and five years	831.7
Gross unrealized gains due between one and five years	1.7
Gross unrealized losses due between one and five years	(0.3)
Estimated fair value due between one and five year	833.1
Estimated fair value, no contractual maturity	14.5
Gross unrealized gains, no contractual maturity	0
Gross unrealized losses, no contractual maturity	(0.2)
Amortized cost, no contractual maturity	14.3
Total investments, amortized cost	2,867.8	2,732.1
Total investments, gross unrealized gains	2.0	1.4
Total investments, gross unrealized losses	(0.5)	(0.8)
Total investments, estimated fair value	$ 2,869.3	$ 2,732.7