Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 2,744.7		$ 2,745.7
Available-for-sale securities, gross unrealized gains	13.3		3.1
Available-for-sale securities, gross unrealized losses	(2.2)		(0.3)
Available-for-sale securities, estimated fair value	2,755.8		2,748.5
Trading securities:
Trading securities, amortized cost	13.8	[1]	12.6	[1]
Trading securities, gross unrealized gains	0	[1]	0	[1]
Trading securities, gross unrealized losses	0	[1]	0	[1]
Trading securities, estimated fair value	13.8	[1]	12.6	[1]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	2,758.5		2,758.3
Total investments, gross unrealized gains	13.3		3.1
Total investments, gross unrealized losses	(2.2)		(0.3)
Total investments, estimated fair value	2,769.6		2,761.1
Cash equivalents [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,108.1		1,225.9
Total investments, gross unrealized gains	0		0
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	1,108.1		1,225.9
Restricted investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	92.2		105.5
Total investments, gross unrealized gains	0		0.1
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	92.2		105.6
Short-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	531.1		441.3
Total investments, gross unrealized gains	12.1		0.3
Total investments, gross unrealized losses	(0.3)		(0.1)
Total investments, estimated fair value	542.9		441.5
Long-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,027.1		985.6
Total investments, gross unrealized gains	1.2		2.7
Total investments, gross unrealized losses	(1.9)		(0.2)
Total investments, estimated fair value	1,026.4		988.1
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,536.8		1,411.3
Available-for-sale securities, gross unrealized gains	1.6		3.0
Available-for-sale securities, gross unrealized losses	(2.0)		(0.2)
Available-for-sale securities, estimated fair value	1,536.4		1,414.1
Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	237.8		226.2
Available-for-sale securities, gross unrealized gains	0.1		0.3
Available-for-sale securities, gross unrealized losses	(0.5)		(0.1)
Available-for-sale securities, estimated fair value	237.4		226.4
Certificate of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	36.0		41.9
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	36.0		41.9
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	6.6		11.6
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	6.6		11.6
Corporate debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	651.9		533.4
Available-for-sale securities, gross unrealized gains	1.3		2.3
Available-for-sale securities, gross unrealized losses	(1.0)		(0.1)
Available-for-sale securities, estimated fair value	652.2		535.6
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	15.8		5.0
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	15.8		5.0
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	292.8		264.6
Available-for-sale securities, gross unrealized gains	0.1		0.3
Available-for-sale securities, gross unrealized losses	(0.5)		0
Available-for-sale securities, estimated fair value	292.4		264.9
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	295.9		328.6
Available-for-sale securities, gross unrealized gains	0.1		0.1
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	296.0		328.7
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	7.6		3.0
Available-for-sale securities, gross unrealized gains	11.7		0
Available-for-sale securities, gross unrealized losses	(0.2)		(0.1)
Available-for-sale securities, estimated fair value	19.1		2.9
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	89.5		102.6
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	89.5		102.6
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	2.7		2.9
Available-for-sale securities, gross unrealized gains	0		0.1
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	2.7		3.0
Restricted investments [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	92.2		105.5
Available-for-sale securities, gross unrealized gains	0		0.1
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	92.2		105.6
Certificate of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	0.3		0.6
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	0.3		0.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost			10.8
Available-for-sale securities, gross unrealized gains			0
Available-for-sale securities, gross unrealized losses			0
Available-for-sale securities, estimated fair value			10.8
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost			6.1
Available-for-sale securities, gross unrealized gains			0
Available-for-sale securities, gross unrealized losses			0
Available-for-sale securities, estimated fair value			6.1
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,011.5		1,042.6
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	1,011.5		1,042.6
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	96.3		165.8
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	96.3		165.8
Cash equivalents [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,108.1		1,225.9
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities, estimated fair value	$ 1,108.1		$ 1,225.9

[1] Balance includes the Company's non-qualified deferred compensation plan assets.