Fair Value Measurements by Balance Sheet Grouping (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents measured at fair value	$ 1,445.3	$ 1,316.2
Short-term investments measured at fair value	191.6	168.9
Long-term investments measured at fair value	291.5	303.9
Restricted investments measured at fair value	79.7	76.1
Prepaid expenses and other current assets measured at fair value	0	0
Total assets measured at fair value	2,008.1	1,865.1
Other accrued liabilities measured at fair value	0	0
Liabilities measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents measured at fair value	196.5	34.5
Short-term investments measured at fair value	202.9	472.4
Long-term investments measured at fair value	541.5	436.8
Restricted investments measured at fair value	1.6	1.6
Prepaid expenses and other current assets measured at fair value	0.4	0.4
Total assets measured at fair value	942.9	945.7
Other accrued liabilities measured at fair value	4.0	9.6
Liabilities measured at fair value	4.0	9.6
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents measured at fair value	0	0
Short-term investments measured at fair value	0	0
Long-term investments measured at fair value	0	0
Restricted investments measured at fair value	0	0
Prepaid expenses and other current assets measured at fair value	0	0
Total assets measured at fair value	0	0
Other accrued liabilities measured at fair value	0	0
Liabilities measured at fair value	0	0
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents measured at fair value	1,641.8	1,350.7
Short-term investments measured at fair value	394.5	641.3
Long-term investments measured at fair value	833.0	740.7
Restricted investments measured at fair value	81.3	77.7
Prepaid expenses and other current assets measured at fair value	0.4	0.4
Total assets measured at fair value	2,951.0	2,810.8
Other accrued liabilities measured at fair value	4.0	9.6
Liabilities measured at fair value	4.0	9.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	2,008.1	1,865.1
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	942.9	945.7
Liabilities measured at fair value	4.0	9.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	0	0
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	2,951.0	2,810.8
Liabilities measured at fair value	$ 4.0	$ 9.6