Real Estate and Accumulated Depreciation (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Real Estate And Accumulated Depreciation [Line Items]
Cost of real estate for federal income tax purposes	$ 10,698
Upper Limit
Real Estate And Accumulated Depreciation [Line Items]
Other properties, percentage	5.00%

Cost basis of real estate investments for federal income tax purposes for entities with a substantial portion of business acquiring and holding investment real estate.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-28.FN6)
-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule III)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Name:	us-gaap_RealEstateFederalIncomeTaxBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant