2. Available-for-Sale Investments (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 373.7
Estimated Fair Value		370.7
Fair Value, Measurements, Recurring [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		$ 1,301.4
Amortized Cost		373.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(3.1)
Estimated Fair Value	[1]	370.7
Available-for-sale Securities, Amortized Cost Basis			418.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			884.8
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			1.5
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		185.7
Estimated Fair Value	[1]	223.6
Fair Value, Measurements, Recurring [Member] | Brokered certificates of deposit [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		0.7
Fair Value, Measurements, Recurring [Member] | Municipal obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		15.0
Estimated Fair Value	[1]	15.6
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		55.6
Estimated Fair Value	[1]	62.9
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		973.4
Fair Value, Measurements, Recurring [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		67.6
Estimated Fair Value	[1]	66.0
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities	[1]		3.4
Estimated Fair Value	[1]	2.6
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			0.2
Amortized Cost		0.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0
Estimated Fair Value		0.2
Available-for-sale Securities, Amortized Cost Basis			0.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Marketable Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			930.0
Available-for-sale Securities, Amortized Cost Basis			54.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			875.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			930.2
Available-for-sale Securities, Amortized Cost Basis			54.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			875.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		0.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0
Estimated Fair Value		0.2
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			371.2
Estimated Fair Value		370.5
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			185.7
Amortized Cost		224.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(1.4)
Estimated Fair Value		223.6
Available-for-sale Securities, Amortized Cost Basis			185.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.5
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Brokered certificates of deposit [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			0.7
Available-for-sale Securities, Amortized Cost Basis			0.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Municipal obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			15.0
Amortized Cost		15.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.1)
Estimated Fair Value		15.6
Available-for-sale Securities, Amortized Cost Basis			15.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.1
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			55.4
Amortized Cost		63.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.4)
Estimated Fair Value		62.7
Available-for-sale Securities, Amortized Cost Basis			55.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.2
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Marketable Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			43.4
Available-for-sale Securities, Amortized Cost Basis			34.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			9.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			371.2
Available-for-sale Securities, Amortized Cost Basis			363.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			9.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			1.5
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			67.6
Amortized Cost		67.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(1.2)
Estimated Fair Value		66.0
Available-for-sale Securities, Amortized Cost Basis			68.3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.7
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			3.4
Amortized Cost		2.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0
Estimated Fair Value		2.6
Available-for-sale Securities, Amortized Cost Basis			3.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax			0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax			0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		373.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(3.1)
Estimated Fair Value		370.5
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities			$ 930.2
Estimated Fair Value		$ 0.2

[1] Available-for-sale investments are included in the following accounts in the Condensed Consolidated Balance Sheets (in millions):   June 30, 2018   December 31, 2017 Short-term investments $ 370.5
  $ 371.2
Other investments 0.2
  930.2
Total $ 370.7
  $ 1,301.4