Summary of Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Percentage of ownership in Swiss franchise	49.00%
Swiss franchise investment portfolio market value	$ 219.9	$ 234.8
Swiss franchise realized gains	12.7	14.7	$ 2.9
Swiss franchise realized losses	2.1	3.8	1.0
Other-than-temporary impairment gains	$ 0.1	$ 1.6	$ 0.3

Other than Temporary Impairment Gains, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities

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- Definition

Investment in debt and equity securities that are categorized as neither held-to-maturity or trading, associated with a Swiss franchise in which we hold a 49 percent interest.

+ References

No definition available.

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- References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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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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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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