Net Income Per Common Share - Average Exercise Prices of Options Excluded from Computation of Diluted Net Income Per Common Share (Detail) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Abstract]
Shares	402,575	180,989	92,738
Average Exercise Price	$ 52.72	$ 54.73	$ 53.17

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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