Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 2,337.2	$ 1,796.4
Available-for-sale securities, gross unrealized gains	4.0	1.1
Available-for-sale securities, gross unrealized losses	(2.3)	(4.6)
Available-for-sale securities, estimated fair value	2,338.9	1,792.9
Trading securities:
Trading securities, amortized cost	19.3	17.7
Trading securities, gross unrealized gains	0.0	0.0
Trading securities, gross unrealized losses	0.0	0.0
Trading securities, estimated fair value	19.3	17.7
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	2,356.5	1,814.1
Total investments, gross unrealized gains	4.0	1.1
Total investments, gross unrealized losses	(2.3)	(4.6)
Total investments, estimated fair value	2,358.2	1,810.6
Cash equivalents [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	555.2	3.4
Total investments, gross unrealized gains	0.0	0.0
Total investments, gross unrealized losses	0.0	0.0
Total investments, estimated fair value	555.2	3.4
Restricted investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	68.8	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	68.8	35.9
Short-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	588.5	527.2
Total investments, gross unrealized gains	0.4	0.9
Total investments, gross unrealized losses	(1.8)	(1.0)
Total investments, estimated fair value	587.1	527.1
Long-term investments [Member]
Available-for-sale and Trading Securities [Abstract]
Total investments, amortized cost	1,144.0	1,247.7
Total investments, gross unrealized gains	3.6	0.1
Total investments, gross unrealized losses	(0.5)	(3.6)
Total investments, estimated fair value	1,147.1	1,244.2
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,786.5	1,751.9
Available-for-sale securities, gross unrealized gains	4.0	0.2
Available-for-sale securities, gross unrealized losses	(0.6)	(3.9)
Available-for-sale securities, estimated fair value	1,789.9	1,748.2
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	320.3	312.2
Available-for-sale securities, gross unrealized gains	0.4	0.0
Available-for-sale securities, gross unrealized losses	0.0	(0.5)
Available-for-sale securities, estimated fair value	320.7	311.7
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	19.4	9.6
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	19.4	9.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	6.7	17.7
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	6.7	17.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	861.3	913.8
Available-for-sale securities, gross unrealized gains	3.0	0.2
Available-for-sale securities, gross unrealized losses	(0.6)	(2.6)
Available-for-sale securities, estimated fair value	863.7	911.4
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	35.5	16.5
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	35.5	16.5
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	177.1	204.1
Available-for-sale securities, gross unrealized gains	0.1	0.0
Available-for-sale securities, gross unrealized losses	0.0	(0.4)
Available-for-sale securities, estimated fair value	177.2	203.7
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	366.2	278.0
Available-for-sale securities, gross unrealized gains	0.5	0.0
Available-for-sale securities, gross unrealized losses	0.0	(0.4)
Available-for-sale securities, estimated fair value	366.7	277.6
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	538.7	29.7
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	538.7	29.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	5.8	6.1
Available-for-sale securities, gross unrealized gains	0.0	0.1
Available-for-sale securities, gross unrealized losses	0.0	0.0
Available-for-sale securities, estimated fair value	5.8	6.2
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, amortized cost	6.2	8.7
Available-for-sale securities, gross unrealized gains	0.0	0.8
Available-for-sale securities, gross unrealized losses	(1.7)	(0.7)
Available-for-sale securities, estimated fair value	$ 4.5	$ 8.8