Cash Equivalents and Investments - Available for Sale Securities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 2,785.1	$ 1,936.4
Available-for-sale Securities, gross unrealized gain	0.7	1.1
Available-for-sale securities, gross unrealized loss	(3.7)	(4.6)
Available-for-sale securities, estimated fair value	2,782.1	1,932.9
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	2,806.1	1,954.1
Total investments, gross unrealized gains	0.7	1.1
Total investments, gross unrealized losses	(3.7)	(4.6)
Total investments, estimated fair value	2,803.1	1,950.6
Cash equivalents [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	907.1	143.4
Total investments, gross unrealized gains	0.0	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	907.1	143.4
Restricted investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	71.9	35.8
Total investments, gross unrealized gains	0.0	0.1
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	71.9	35.9
Restricted investments [Member] | Prepaid Expenses and Other Current Assets [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, estimated fair value	4.0
Short-term investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	753.4	527.2
Total investments, gross unrealized gains	0.1	0.9
Total investments, gross unrealized losses	(1.2)	(1.0)
Total investments, estimated fair value	752.3	527.1
Long-term investments [Member]
Available-for-sale and Trading Investments [Abstract]
Total investments, amortized cost	1,073.7	1,247.7
Total investments, gross unrealized gains	0.6	0.1
Total investments, gross unrealized losses	(2.5)	(3.6)
Total investments, estimated fair value	1,071.8	1,244.2
Fixed Income Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	2,179.6	1,891.9
Available-for-sale Securities, gross unrealized gain	0.7	0.2
Available-for-sale securities, gross unrealized loss	(3.0)	(3.9)
Available-for-sale securities, estimated fair value	2,177.3	1,888.2
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	303.0	312.2
Available-for-sale Securities, gross unrealized gain	0.2	0.0
Available-for-sale securities, gross unrealized loss	(0.2)	(0.5)
Available-for-sale securities, estimated fair value	303.0	311.7
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	66.1	9.6
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	66.1	9.6
Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	147.7	17.7
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	147.7	17.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	846.5	913.8
Available-for-sale Securities, gross unrealized gain	0.4	0.2
Available-for-sale securities, gross unrealized loss	(2.0)	(2.6)
Available-for-sale securities, estimated fair value	844.9	911.4
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	34.0	16.5
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.1)	0.0
Available-for-sale securities, estimated fair value	33.9	16.5
Time deposits [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	264.6	140.0
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	264.6	140.0
U.S. government agency securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	127.0	204.1
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.3)	(0.4)
Available-for-sale securities, estimated fair value	126.7	203.7
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	390.7	278.0
Available-for-sale Securities, gross unrealized gain	0.1	0.0
Available-for-sale securities, gross unrealized loss	(0.4)	(0.4)
Available-for-sale securities, estimated fair value	390.4	277.6
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	592.2	29.7
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	592.2	29.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	8.0	6.1
Available-for-sale Securities, gross unrealized gain	0.0	0.1
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities, estimated fair value	8.0	6.2
Trading securities:
Trading securities, amortized cost	21.0	17.7
Trading securities, gross unrealized gain	0.0	0.0
Trading securities, gross unrealized loss	0.0	0.0
Trading securities, estimated fair value	21.0	17.7
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	5.3	8.7
Available-for-sale Securities, gross unrealized gain	0.0	0.8
Available-for-sale securities, gross unrealized loss	(0.7)	(0.7)
Available-for-sale securities, estimated fair value	$ 4.6	$ 8.8