Investments and Fair Value Measurements (Gains (Losses) on Investments) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale:
Available-for-sale securities, amortized cost basis	$ 22.7	$ 20.9
Available-for-sale securities, unrealized gain	3.1	1.2
Available-for-sale securities, unrealized loss	0.0	(2.0)
Available-for-sale securities, current	25.8	20.1
Held-to-maturity:
Held-to-maturity securities, total amortized cost	2.3	2.5
Held-to-maturity securities, unrealized gain	0.0	0.0
Held-to-maturity securities, unrealized loss	0.0	0.0
Held-to-maturity securities, current	2.3	2.5
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]
Available-for-sale securities, realized gains	1.2	1.8	$ 3.4
Available-for-sale securities, realized losses	0.0	(0.8)	(0.2)
Available-for-sale securities, realized gains, net	1.2	1.0	$ 3.2
Equity securities and exchange-traded funds
Available-for-sale:
Available-for-sale securities, amortized cost basis	19.0	17.9
Available-for-sale securities, unrealized gain	2.9	1.2
Available-for-sale securities, unrealized loss	0.0	(1.8)
Available-for-sale securities, fair value disclosure	21.9	17.3
Mutual funds
Available-for-sale:
Available-for-sale securities, amortized cost basis	3.7	3.0
Available-for-sale securities, unrealized gain	0.2	0.0
Available-for-sale securities, unrealized loss	0.0	(0.2)
Available-for-sale securities, fair value disclosure	3.9	2.8
Certificates of deposit [Member]
Held-to-maturity:
Held-to-maturity securities, total amortized cost	2.3	2.5
Held-to-maturity securities, unrealized gain	0.0	0.0
Held-to-maturity securities, unrealized loss	0.0	0.0
Held-to-maturity securities, current	$ 2.3	$ 2.5

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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Amount of investment in debt security measured at amortized cost (held-to-maturity), classified as current.

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