Investments and Fair Value Measurements (Gains (Losses) on Investments) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Available-for-sale:
Available-for-sale securities, Cost	$ 20.2		$ 19.5
Available-for-sale Securities, Unrealized Gain	2.1		2.6
Available-for-sale Securities, Unrealized Loss	(1.0)		(0.6)
Available-for-sale securities, Fair Value	21.3		21.5
Held-to-maturity:
Held-to-maturity securities, Cost	21.5		21.9
Held-to-maturity Securities, Unrecognized Gain	0.0		0.0
Held-to-maturity Securities, Unrecognized Loss	0.0		0.0
Held-to-maturity	21.5		21.9
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Available-for-sale securities, realized gains	0.6	$ 0.5
Available-for-sale securities, realized losses	(0.1)	0.0
Available-for-sale securities, realized gains (losses), net	0.5	$ 0.5
Equity Securities and Exchange-traded Funds [Member]
Available-for-sale:
Available-for-sale securities, Cost	17.8		17.1
Available-for-sale Securities, Unrealized Gain	2.0		2.4
Available-for-sale Securities, Unrealized Loss	(1.0)		(0.6)
Available-for-sale securities, Fair Value	18.8		18.9
Mutual Funds [Member]
Available-for-sale:
Available-for-sale securities, Cost	2.4		2.4
Available-for-sale Securities, Unrealized Gain	0.1		0.2
Available-for-sale Securities, Unrealized Loss	0.0		0.0
Available-for-sale securities, Fair Value	2.5		2.6
Certificates of Deposit [Member]
Held-to-maturity:
Held-to-maturity securities, Cost	19.5		19.9
Held-to-maturity Securities, Unrecognized Gain	0.0		0.0
Held-to-maturity Securities, Unrecognized Loss	0.0		0.0
Held-to-maturity	19.5		19.9
Convertible Debt Securities [Member]
Held-to-maturity:
Held-to-maturity securities, Cost	2.0		2.0
Held-to-maturity Securities, Unrecognized Gain	0.0		0.0
Held-to-maturity Securities, Unrecognized Loss	0.0		0.0
Held-to-maturity	$ 2.0		$ 2.0

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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.

+ References

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-Publisher FASB
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-Section 50
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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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-Publisher FASB
-Name Accounting Standards Codification
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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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-Publisher FASB
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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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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle.

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