Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 2,775.0		$ 2,745.7
Available-for-sale securities, gross unrealized gains	113.5		3.1
Available-for-sale securities, gross unrealized losses	(1.0)		(0.3)
Available-for-sale securities, estimated fair value	2,887.5		2,748.5
Trading securities:
Trading securities, amortized cost	15.1	[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	15.1	[1]	12.6	[1]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	2,790.1		2,758.3
Total investments, gross unrealized gains	113.5		3.1
Total investments, gross unrealized losses	(1.0)		(0.3)
Total investments, estimated fair value	2,902.6		2,761.1
Cash equivalents [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,049.7		1,225.9
Total investments, gross unrealized gains	0		0
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	1,049.7		1,225.9
Restricted investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	90.5		105.5
Total investments, gross unrealized gains	0.1		0.1
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	90.6		105.6
Short-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	461.7		441.3
Total investments, gross unrealized gains	112.0		0.3
Total investments, gross unrealized losses	0		(0.1)
Total investments, estimated fair value	573.7		441.5
Long-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,188.2		985.6
Total investments, gross unrealized gains	1.4		2.7
Total investments, gross unrealized losses	(1.0)		(0.2)
Total investments, estimated fair value	1,188.6		988.1
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,619.6		1,411.3
Available-for-sale securities, gross unrealized gains	1.9		3.0
Available-for-sale securities, gross unrealized losses	(1.0)		(0.2)
Available-for-sale securities, estimated fair value	1,620.5		1,414.1
Asset-backed securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	253.8		226.2
Available-for-sale securities, gross unrealized gains	0.1		0.3
Available-for-sale securities, gross unrealized losses	(0.2)		(0.1)
Available-for-sale securities, estimated fair value	253.7		226.4
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	24.8		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	24.8		41.9
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	11.5		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	11.5		11.6
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	768.2		533.4
Available-for-sale securities, gross unrealized gains	1.5		2.3
Available-for-sale securities, gross unrealized losses	(0.6)		(0.1)
Available-for-sale securities, estimated fair value	769.1		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	315.1		264.6
Available-for-sale securities, gross unrealized gains	0.2		0.3
Available-for-sale securities, gross unrealized losses	(0.2)		0
Available-for-sale securities, estimated fair value	315.1		264.9
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	230.4		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	230.5		328.7
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	15.2		3.0
Available-for-sale securities, gross unrealized gains	111.5		0
Available-for-sale securities, gross unrealized losses	0		(0.1)
Available-for-sale securities, estimated fair value	126.7		2.9
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	88.1		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	88.1		102.6
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	2.4		2.9
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	2.5		3.0
Restricted investments [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	90.5		105.5
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	90.6		105.6
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	0.1		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	0.1		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	8.6		6.1
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	8.6		6.1
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	956.0		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	956.0		1,042.6
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	85.0		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	85.0		165.8
Cash equivalents [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,049.7		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	$ 1,049.7		$ 1,225.9

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