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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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Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Revenue (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Loyalty program, points [Member]
Revenue
Deferred revenue, period expected to be recognized within	12 months		12 months
Loyalty program, coupon [Member]
Revenue
Deferred revenue, period expected to be recognized within	3 months		3 months
Loyalty program [Member]
Revenue
Deferred revenue	$ 5.0		$ 5.0		$ 4.7
Deferred revenue, recognized	$ 4.3	$ 4.6	$ 11.8	$ 12.9