Schedule of Concentration Risk of Lease Rental Income (Detail) - Lease Rental Income - Customer Concentration Risk	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Customer A
Concentration Risk [Line Items]
Percentage of lease rental income	18.40%	15.40%	14.00%
Customer B
Concentration Risk [Line Items]
Percentage of lease rental income	12.70%	13.30%	13.40%
Customer C
Concentration Risk [Line Items]
Percentage of lease rental income	9.40%	8.30%	8.40%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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