Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 2,088.6		$ 2,777.8
Available-for-sale securities, gross unrealized gains	1.5		106.9
Available-for-sale securities, gross unrealized losses	(1.0)		(2.5)
Available-for-sale securities, estimated fair value	2,089.1		2,882.2
Trading securities:
Trading securities, amortized cost	16.1	[1]	15.4	[1]
Trading securities, gross unrealized gains	0	[1]	0	[1]
Trading securities, gross unrealized losses	0	[1]	0	[1]
Trading securities, estimated fair value	16.1	[1]	15.4	[1]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	2,104.7		2,793.2
Total investments, gross unrealized gains	1.5		106.9
Total investments, gross unrealized losses	(1.0)		(2.5)
Total investments, estimated fair value	2,105.2		2,897.6
Cash equivalents [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,135.1		996.2
Total investments, gross unrealized gains	0		0
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	1,135.1		996.2
Restricted investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	70.7		87.5
Total investments, gross unrealized gains	0.1		0.1
Total investments, gross unrealized losses	0		0
Total investments, estimated fair value	70.8		87.6
Short-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	378.6		459.0
Total investments, gross unrealized gains	0.3		104.9
Total investments, gross unrealized losses	(0.8)		(2.0)
Total investments, estimated fair value	378.1		561.9
Long-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	520.3		1,250.5
Total investments, gross unrealized gains	1.1		1.9
Total investments, gross unrealized losses	(0.2)		(0.5)
Total investments, estimated fair value	521.2		1,251.9
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	894.4		1,718.2
Available-for-sale securities, gross unrealized gains	1.4		2.3
Available-for-sale securities, gross unrealized losses	(0.2)		(0.6)
Available-for-sale securities, estimated fair value	895.6		1,719.9
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	135.4		249.9
Available-for-sale securities, gross unrealized gains	0		0.1
Available-for-sale securities, gross unrealized losses	(0.1)		(0.1)
Available-for-sale securities, estimated fair value	135.3		249.9
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	12.0		27.6
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	12.0		27.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1.2		6.9
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	1.2		6.9
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	411.0		813.6
Available-for-sale securities, gross unrealized gains	1.3		2.0
Available-for-sale securities, gross unrealized losses	(0.1)		(0.3)
Available-for-sale securities, estimated fair value	412.2		815.3
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	5.0		10.7
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	5.0		10.7
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	88.3		306.2
Available-for-sale securities, gross unrealized gains	0		0.1
Available-for-sale securities, gross unrealized losses	0		(0.1)
Available-for-sale securities, estimated fair value	88.3		306.2
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	241.5		303.3
Available-for-sale securities, gross unrealized gains	0.1		0.1
Available-for-sale securities, gross unrealized losses	0		(0.1)
Available-for-sale securities, estimated fair value	241.6		303.3
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,183.0		1,043.7
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	1,183.0		1,043.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	4.0		3.9
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	4.1		4.0
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	7.2		12.0
Available-for-sale securities, gross unrealized gains	0		104.5
Available-for-sale securities, gross unrealized losses	(0.8)		(1.9)
Available-for-sale securities, estimated fair value	$ 6.4		$ 114.6

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