Benefit Plans - Summary of Company's Restricted Stock Units Activity (Detail) - Restricted Stock Units [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning Balance	139,510	93,664	125,240
Number of Shares, granted	83,440	97,260	70,960
Number of Shares, vested	(25,862)	(30,214)	(36,336)
Number of Shares, cancelled/forfeited/expired	(840)	(21,200)	(66,200)
Number of Shares, Ending Balance	196,248	139,510	93,664
Weighted-Average Grant Date Fair Value, Beginning Balance	$ 44.73	$ 33.62	$ 30.66
Weighted-Average Grant Date Fair Value, granted	59.98	49.47	34.53
Weighted-Average Grant Date Fair Value, vested	48.31	33.16	26.99
Weighted-Average Grant Date Fair Value, cancelled/forfeited/expired	59.84	33.88	32.63
Weighted-Average Grant Date Fair Value, Ending Balance	$ 50.68	$ 44.73	$ 33.62
Aggregate Intrinsic Value	$ 15.0

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
-Paragraph 2
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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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 2
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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).

+ References

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-Publisher FASB
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- Definition

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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
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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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-Publisher FASB
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- Definition

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.

+ References

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-Publisher FASB
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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.

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No definition available.

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

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