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Fair Value Measurements and Investments Available-for-Sale Investments (Details) - USD ($) $ in Millions		Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	$ 1,136.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	1,142.2
Fair Value, Recurring [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale		1,136.0	$ 1,300.0
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	531.6	699.3
Fair Value, Recurring [Member] | Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	12.1	23.1
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	323.7	333.4
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	255.9	230.7
Fair Value, Recurring [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	12.7	13.5
Short-term Investments [Member] | Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	531.6	699.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(3.3)	(10.8)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	0.8	0.2
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	534.1	709.9
Short-term Investments [Member] | Fair Value, Recurring [Member] | Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	12.1	23.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(0.1)	(0.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	0.0	0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	12.2	23.4
Short-term Investments [Member] | Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	323.7	333.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(2.7)	(6.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	0.7	0.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	325.7	339.6
Short-term Investments [Member] | Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	255.9	230.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(1.6)	(3.2)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	0.1	0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	257.4	233.9
Short-term Investments [Member] | Fair Value, Recurring [Member] | Foreign Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	12.7	13.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(0.1)	(0.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	0.0	0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	12.8	13.8
Short-term Investments [Member] | Fair Value, Recurring [Member] | Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	[1]	1,136.0	1,300.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	[1]	(7.8)	(20.9)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	[1]	1.6	0.3
Debt Securities, Available-for-sale, Allowance for Credit Loss	[1]	0.0
Debt Securities, Available-for-sale, Amortized Cost	[1]	$ 1,142.2	$ 1,320.6
[1] Available-for-sale investments are included in Short-term investments in the consolidated balance sheets.

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