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Share-based compensation arrangement by share-based payment award, options, grants in period, vest percentage per annum.

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Share-based Compensation Arrangement by Share-based Payment Award, Purchase Offering Period

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Share-based compensation arrangement by share-based payment award, requisite service period begins and continue to vest, employment continuation period.

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Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, Grants in Period, Performance Target, Percentage

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Unrecognized cost of unvested options awarded to employees as compensation.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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 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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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.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Percentage of vesting of share-based compensation awards.

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Intrinsic value of equity-based compensation awards vested. Excludes stock and unit options.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Purchase price of common stock expressed as a percentage of its fair value.

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Equity Awards - Narrative (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018 USD ($) incentive_plan shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity incentive plans \| incentive_plan		2
Total intrinsic value of options exercised		$ 204,337	$ 277,670	$ 157,800
Weighted-average grant date fair value of options granted (in USD per share) \| $ / shares			$ 37.57	$ 28.01
Number of shares, granted (in shares) \| shares		0	616,720
Fair value of stock options vested		$ 12,300	$ 11,800	$ 17,000
Total unrecognized compensation cost, adjusted for estimated forfeitures, related to unvested stock options		$ 11,700
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average remaining vesting period		2 years 3 months 25 days
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average remaining vesting period		2 years 9 months 29 days
Number of shares outstanding \| shares		10,201,660	11,403,341	12,222,282
Aggregate intrinsic value, vested		$ 931,848	$ 573,861
Performance target (in percent)		100.00%
Unrecognized compensation expense expected to be recognized		$ 871,400
Restricted stock units with service condition only
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding \| shares		9,443,982
Aggregate intrinsic value, vested				$ 354,300
Vesting term (in years)		4 years
Restricted Stock Units With Service And Performance Conditions
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding \| shares		757,678
Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term (in years)		3 years
Performance target (in percent)		100.00%
Allocated share-based compensation expense		$ 91,800	$ 40,500	$ 36,100
Tranche one \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term (in years)		3 months
Award vesting percentage		33.00%
Tranche two \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term (in years)	2 years	3 months
Tranche three \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term (in years)		3 months
Tranche four \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term (in years)		3 months
Minimum \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance target (in percent)		0.00%
Maximum \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance target (in percent)		180.00%
President And Chief Executive Officer \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares) \| shares			396,720
2012 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock outstanding, increase, percentage		5.00%
2012 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock outstanding, increase, percentage		1.00%
Common stock purchase price, percentage		85.00%
Offering period		6 months
2005 Stock Plan and 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted to new employees vest, percentage per annum		25.00%
Requisite service period to vest employment continuation period		3 years
Options granted, exercisable period		10 years