Stock-Based Compensation - Disclosures for 2004 Purchase Plan (Detail) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Weighted-average purchase price per share (in dollars per share)	$ 371.71	$ 354.72	$ 341.48
Weighted-average grant date fair value per share (in dollars per share)	$ 114.08	$ 104.84	$ 90.04
Number of shares purchased (in shares)	167,113	146,640	145,346

Number of shares issued during the period as a result of an employee stock purchase plan.

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Name:	us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration