Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 168.2		$ 568.7
Unrealized loss, less than 12 months	(1.0)		(2.5)
Fair value, 12 months or greater	11.0		10.3
Unrealized loss, 12 months or greater	0		0
Total fair value, available-for-sale investments in continuous unrealized loss position	179.2		579.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.0)		(2.5)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	161.8		561.9
Unrealized loss, less than 12 months	(0.2)		(0.6)
Fair value, 12 months or greater	11.0		10.3
Unrealized loss, 12 months or greater	0		0
Total fair value, available-for-sale investments in continuous unrealized loss position	172.8		572.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.2)		(0.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	65.9	[1]	153.0	[2]
Unrealized loss, less than 12 months	(0.1)	[1]	(0.1)	[2]
Fair value, 12 months or greater	6.0	[1]	0.6	[2]
Unrealized loss, 12 months or greater	0	[1]	0	[2]
Total fair value, available-for-sale investments in continuous unrealized loss position	71.9	[1]	153.6	[2]
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.1)	[1]	(0.1)	[2]
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	54.1		156.1	[2]
Unrealized loss, less than 12 months	(0.1)		(0.3)	[2]
Fair value, 12 months or greater	0		9.7	[2]
Unrealized loss, 12 months or greater	0		0	[2]
Total fair value, available-for-sale investments in continuous unrealized loss position	54.1		165.8	[2]
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.1)		(0.3)	[2]
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	0	[1]	10.0	[3]
Unrealized loss, less than 12 months	0	[1]	0	[3]
Fair value, 12 months or greater	5.0	[1]	0	[3]
Unrealized loss, 12 months or greater	0	[1]	0	[3]
Total fair value, available-for-sale investments in continuous unrealized loss position	5.0	[1]	10.0	[3]
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0	[1]	0	[3]
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	34.9	[4]	123.1
Unrealized loss, less than 12 months	0	[4]	(0.1)
Fair value, 12 months or greater	0	[4]	0
Unrealized loss, 12 months or greater	0	[4]	0
Total fair value, available-for-sale investments in continuous unrealized loss position	34.9	[4]	123.1
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0	[4]	(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	6.9	[4]	119.7
Unrealized loss, less than 12 months	0	[4]	(0.1)
Fair value, 12 months or greater	0	[4]	0
Unrealized loss, 12 months or greater	0	[4]	0
Total fair value, available-for-sale investments in continuous unrealized loss position	6.9	[4]	119.7
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0	[4]	(0.1)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	6.4		6.8
Unrealized loss, less than 12 months	(0.8)		(1.9)
Fair value, 12 months or greater	0		0
Unrealized loss, 12 months or greater	0		0
Total fair value, available-for-sale investments in continuous unrealized loss position	6.4		6.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	$ (0.8)		$ (1.9)

[1] Balances greater than 12 months include investments that were in an immaterial unrealized loss position as of March 31, 2014.

[2] Balances greater than 12 months include investments that were in an immaterial unrealized loss position as of December 31, 2013.

[3] Balances less than 12 months include investments that were in an immaterial unrealized loss position as of December 31, 2013.

[4] Balances less than 12 months include investments that were in an immaterial unrealized loss position as of March 31, 2014.