3. Available-for-Sale Investments (Details) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 368.4
Debt Securities, Available-for-sale		370.9
Fair Value, Recurring [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	370.9	$ 373.2
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	181.7	215.0
Fair Value, Recurring [Member] | Municipal obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	10.7	11.0
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	98.5	80.3
Fair Value, Recurring [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	4.7	3.6
Fair Value, Recurring [Member] | Foreign Government Debt [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	3.1
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	72.2	63.3
Fair Value, Recurring [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		368.4	375.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		2.8	0.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.3)	(2.6)
Debt Securities, Available-for-sale		370.9	373.2
Fair Value, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		0.1	0.2
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
Debt Securities, Available-for-sale		0.1	0.2
Fair Value, Recurring [Member] | Other Long-term Investments [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		0.1	0.2
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
Debt Securities, Available-for-sale		0.1	0.2
Fair Value, Recurring [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	370.8	373.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		180.3	216.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		1.5	0.1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.1)	(1.3)
Debt Securities, Available-for-sale		181.7	215.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Municipal obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		10.6	11.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.1	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0	(0.1)
Debt Securities, Available-for-sale		10.7	11.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		97.6	80.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		1.0	0.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.1)	(0.8)
Debt Securities, Available-for-sale		98.5	80.3
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4.7	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
Debt Securities, Available-for-sale		4.7	3.6
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Debt [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		3.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0.0
Debt Securities, Available-for-sale		3.1
Fair Value, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		72.0	63.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0.2	0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.1)	(0.4)
Debt Securities, Available-for-sale		72.1	63.1
Fair Value, Recurring [Member] | Short-term Investments [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		368.3	375.3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		2.8	0.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(0.3)	(2.6)
Debt Securities, Available-for-sale		370.8	373.0
Fair Value, Recurring [Member] | Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	$ 0.1	$ 0.2

[1] Available-for-sale investments are included in the following accounts in the Condensed Consolidated Balance Sheets (in millions):   September 30, 2019   December 31, 2018 Short-term investments $ 370.8
  $ 373.0
Other investments 0.1
  0.2
Total $ 370.9
  $ 373.2