Share-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) yr shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted awards (shares)	751
Share-based Payment Arrangement, Shares Withheld for Tax Withholding Obligation	7,635	16,918
Payment, Tax Withholding, Share-based Payment Arrangement \| $	$ 692	$ 1,210
Unrecognized compensation cost \| $	$ 4,300
Awards to non-employee directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value \| $ / shares	$ 93.14
Amortization of expense equally over a service period	1 year
Granted awards (shares)	6,759
Unrecognized compensation expense related to the awards to non-employee directors \| $	$ 400
Awards to non-employee director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value \| $ / shares	$ 75.70
Granted awards (shares)	276
Award to key employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expired In Period	15,086
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value \| $ / shares	$ 94.34
Granted awards (shares)	45,016
Vesting period	3 years
Payment, Tax Withholding, Share-based Payment Arrangement \| $	$ 700
Intrinsic value of the SICP awards \| $	$ 12,900
Accelerated Vested Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Shares Withheld for Tax Withholding Obligation	7,635
Accelerated Vested Shares [Member] \| Award to key employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number Of Multi year Awards \| yr		3
Share-based Compensation Arrangement by Share-based Payment Award, Accelerated Vesting, Number		14,107
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expired In Period		2,569
Share-based Payment Arrangement, Accelerated Cost \| $		$ 1,100

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Period for amortization of share based expenses.

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Share based compensation arrangement by share based payment award equity instruments other than options expired in period.

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
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Amount of additional cost recognized for award under share-based payment arrangement from occurrence of event accelerating recognition of cost.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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- Definition

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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- Definition

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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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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Number of shares for which recognition of cost was accelerated for award under share-based payment arrangement.

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Intrinsic value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Number of shares used to settle grantee's tax withholding obligation for award under share-based payment arrangement.

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