Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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-Section 45
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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+ References

No definition available.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
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Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Revenue (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Loyalty program, points [Member]
Revenue
Deferred revenue, period expected to be recognized within	12 months
Loyalty program, coupon [Member]
Revenue
Deferred revenue, period expected to be recognized within	3 months
Loyalty program [Member]
Revenue
Deferred revenue	$ 4.6		$ 4.7
Deferred revenue, recognized	$ 3.2	$ 3.4