Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 3,283.1	$ 2,785.1
Available-for-sale securities, gross unrealized gains	2.5	0.7
Available-for-sale securities, gross unrealized losses	(2.8)	(3.7)
Available-for-sale securities, estimated fair value	3,282.8	2,782.1
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	3,305.7	2,806.1
Total investments, gross unrealized gains	2.5	0.7
Total investments, gross unrealized losses	(2.8)	(3.7)
Total investments, estimated fair value	3,305.4	2,803.1
Cash equivalents [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,429.9	907.1
Total investments, gross unrealized gains	0.0	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	1,429.9	907.1
Restricted investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	73.8	71.9
Total investments, gross unrealized gains	0.0	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	73.8	71.9
Short-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	741.0	753.4
Total investments, gross unrealized gains	1.8	0.1
Total investments, gross unrealized losses	(0.5)	(1.2)
Total investments, estimated fair value	742.3	752.3
Long-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,061.0	1,073.7
Total investments, gross unrealized gains	0.7	0.6
Total investments, gross unrealized losses	(2.3)	(2.5)
Total investments, estimated fair value	1,059.4	1,071.8
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	2,152.9	2,179.6
Available-for-sale securities, gross unrealized gains	0.9	0.7
Available-for-sale securities, gross unrealized losses	(2.8)	(3.0)
Available-for-sale securities, estimated fair value	2,151.0	2,177.3
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	314.6	303.0
Available-for-sale securities, gross unrealized gains	0.0	0.2
Available-for-sale securities, gross unrealized losses	(0.4)	(0.2)
Available-for-sale securities, estimated fair value	314.2	303.0
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	57.4	66.1
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	57.4	66.1
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	125.9	147.7
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	125.9	147.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	856.1	846.5
Available-for-sale securities, gross unrealized gains	0.8	0.4
Available-for-sale securities, gross unrealized losses	(1.5)	(2.0)
Available-for-sale securities, estimated fair value	855.4	844.9
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	40.0	34.0
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	(0.1)	(0.1)
Available-for-sale securities, estimated fair value	39.9	33.9
Time deposits [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	245.8	264.6
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	245.8	264.6
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	132.4	127.0
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	(0.4)	(0.3)
Available-for-sale securities, estimated fair value	132.0	126.7
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	380.7	390.7
Available-for-sale securities, gross unrealized gains	0.1	0.1
Available-for-sale securities, gross unrealized losses	(0.4)	(0.4)
Available-for-sale securities, estimated fair value	380.4	390.4
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,117.3	592.2
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	1,117.3	592.2
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	8.3	8.0
Available-for-sale securities, gross unrealized gains	0.0	0.0
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	8.3	8.0
Trading securities:
Trading securities, amortized cost	22.6	21.0
Trading securities, gross unrealized gains	0.0	0.0
Trading securities, gross unrealized losses	0.0	0.0
Trading securities, estimated fair value	22.6	21.0
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	4.6	5.3
Available-for-sale securities, gross unrealized gains	1.6	0.0
Available-for-sale securities, gross unrealized losses	0.0	(0.7)
Available-for-sale securities, estimated fair value	$ 6.2	$ 4.6