Cash Equivalents and Investments - Available for Sale Securities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 1,796.4	$ 2,071.9
Available-for-sale Securities, gross unrealized gain	1.1	0.6
Available-for-sale securities, gross unrealized loss	(4.6)	(1.7)
Available-for-sale securities, estimated fair value	1,792.9	2,070.8
Trading securities:
Trading securities, amortized cost	17.7	16.3
Trading securities, gross unrealized gain	0.0	0.0
Trading securities, gross unrealized loss	0.0	0.0
Trading securities, estimated fair value	17.7	16.3
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	1,814.1	2,088.2
Total investments, gross unrealized gains	1.1	0.6
Total investments, gross unrealized losses	(4.6)	(1.7)
Total investments, estimated fair value	1,810.6	2,087.1
Cash equivalents [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	3.4	576.6
Total investments, gross unrealized gains	0.0	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	3.4	576.6
Restricted investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	35.8	45.2
Total investments, gross unrealized gains	0.1	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	35.9	45.2
Short-term investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	527.2	332.2
Total investments, gross unrealized gains	0.9	0.2
Total investments, gross unrealized losses	(1.0)	(0.2)
Total investments, estimated fair value	527.1	332.2
Long-term investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	1,247.7	1,134.2
Total investments, gross unrealized gains	0.1	0.4
Total investments, gross unrealized losses	(3.6)	(1.5)
Total investments, estimated fair value	1,244.2	1,133.1
Fixed Income Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,751.9	1,471.7
Available-for-sale Securities, gross unrealized gain	0.2	0.5
Available-for-sale securities, gross unrealized loss	(3.9)	(1.6)
Available-for-sale securities, estimated fair value	1,748.2	1,470.6
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	312.2	269.3
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.5)	(0.3)
Available-for-sale securities, estimated fair value	311.7	269.0
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	9.6	10.6
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	9.6	10.6
Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	17.7	20.3
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	17.7	20.3
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	913.8	738.6
Available-for-sale Securities, gross unrealized gain	0.2	0.5
Available-for-sale securities, gross unrealized loss	(2.6)	(1.1)
Available-for-sale securities, estimated fair value	911.4	738.0
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	16.5	24.6
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	16.5	24.6
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	204.1	162.2
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.4)	(0.1)
Available-for-sale securities, estimated fair value	203.7	162.1
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	278.0	246.1
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.4)	(0.1)
Available-for-sale securities, estimated fair value	277.6	246.0
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	29.7	594.2
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	29.7	594.2
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	6.1	3.9
Available-for-sale Securities, gross unrealized gain	0.1	0.1
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	6.2	4.0
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	8.7	2.1
Available-for-sale Securities, gross unrealized gain	0.8	0.0
Available-for-sale securities, gross unrealized loss	(0.7)	(0.1)
Available-for-sale securities, estimated fair value	$ 8.8	$ 2.0