3. Fair Value Measurements Short-term Investments (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 418.1	$ 435.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		884.8	681.7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(1.5)	(1.8)
Estimated Fair Value	[1]	1,301.4	1,115.6
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	185.7	179.4
Interest-bearing Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		0.7	3.6
Municipal Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	15.0	15.4
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	55.6	62.5
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	973.4	767.8
US Government Sponsored Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	67.6	82.5
Foreign Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	3.4	4.4
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		371.2	383.2
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		185.9	179.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.3	0.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.5)	(0.5)
Estimated Fair Value		185.7	179.4
Short-term Investments [Member] | Interest-bearing Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		0.7	3.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	0.0
Estimated Fair Value		0.7	3.6
Short-term Investments [Member] | Municipal Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		15.1	15.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.1)	(0.1)
Estimated Fair Value		15.0	15.4
Short-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		55.6	62.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.2)	(0.1)
Estimated Fair Value		55.4	62.2
Short-term Investments [Member] | Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		34.4	32.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		9.0	3.7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	(0.4)
Estimated Fair Value		43.4	35.7
Short-term Investments [Member] | Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		363.4	380.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		9.3	4.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(1.5)	(1.8)
Estimated Fair Value		371.2	383.2
Short-term Investments [Member] | US Government Sponsored Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		68.3	83.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.7)	(0.7)
Estimated Fair Value		67.6	82.5
Short-term Investments [Member] | Foreign Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3.4	4.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	0.0
Estimated Fair Value		3.4	4.4
Other Long-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		0.2	0.3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	0.0
Estimated Fair Value		0.2	0.3
Other Long-term Investments [Member] | Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		54.5	54.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		875.5	677.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	0.0
Estimated Fair Value		930.0	732.1
Other Long-term Investments [Member] | Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		54.7	54.8
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		875.5	677.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	0.0
Estimated Fair Value		$ 930.2	$ 732.4

[1] Available-for-sale investments are included in the following accounts in the Consolidated Balance Sheets (in millions):   December 31,
2017   December 31, 2016         Short-term investments $ 371.2
  $ 383.2
Other investments 930.2
  732.4
Total $ 1,301.4
  $ 1,115.6