Significant Accounting Policies - Marketable and Non Marketable Securities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Marketable and Non-Marketable Securities
Net unrealized gains recorded in other comprehensive income (loss)	$ 16.1	$ 15.4
Carrying value of investments under the cost method	211.0	210.5
Available for sale securities
Marketable and Non-Marketable Securities
Marketable Securities	$ 161.9	$ 156.2
Available for sale securities \| Securities in captive insurance subsidiary portfolio \| Minimum
Marketable and Non-Marketable Securities
Investment maturity period	1 year
Available for sale securities \| Securities in captive insurance subsidiary portfolio \| Maximum
Marketable and Non-Marketable Securities
Investment maturity period	10 years

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No definition available.

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- Definition

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669686-108580

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- Definition

Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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