Stock-based Compensation (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	750,300		775,900		727,700	750,900	750,900
Granted (in shares)	5,800		5,100		225,800	157,000
Vested (in shares)	(81,200)		(70,900)		(259,300)	(177,300)
Forfeited (in shares)	(35,500)		(800)		(54,800)	(21,300)
Unvested at end of period (in shares)	639,400	750,300	709,300	775,900	639,400	709,300	727,700	750,900
Restricted stock unit activity, additional disclosures [Abstract]
Weighted average grant date fair value, beginning of period (in dollars per share)	$ 131.09		$ 117.74		$ 118.96	$ 113.97	$ 113.97
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	144.43		115.47		161.87	116.50
Weighted average grant date fair value, vested (in dollars per share)	123.08		112.64		122.63	99.24
Weighted average grant date fair value, end of period (in dollars per share)	$ 131.93	$ 131.09	$ 118.22	$ 117.74	$ 131.93	$ 118.22	$ 118.96	$ 113.97
Weighted average remaining contractual life, unvested shares outstanding	2 years 2 months 16 days	2 years 4 months 10 days	1 year 5 months 6 days	1 year 7 months 10 days	2 years 2 months 16 days	1 year 5 months 6 days	1 year 2 months 26 days	1 year 8 months 15 days
Unamortized deferred compensation cost	$ 34.7				$ 34.7
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value	$ 136.07		$ 130.32		$ 132.64	$ 113.18

Unrecognized cost of unvested share-based compensation awards.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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