The capitalized costs incurred during the period (excluded from amortization) to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-10.(c)(7)(ii))

-URI http://asc.fasb.org/extlink&oid=21918352&loc=d3e511914-122862

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 10

-Paragraph c

-Subparagraph 3(ii)(A)

-Article 4

Name:	us-gaap_AcquisitionCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Financial Information by Business Segment - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Oct. 31, 2012 MW Company	Jul. 31, 2012 MW	Jul. 31, 2012 Contingent consideration [Member]	Oct. 31, 2012 Deposits relating to power purchase agreements [Member]	Jul. 31, 2012 Deposits relating to power purchase agreements [Member]	Oct. 31, 2012 Construction [Member]	Jul. 31, 2012 Construction [Member]	Oct. 31, 2012 Construction work in progress [Member]	Dec. 31, 2012 Con Edison Development [Member]
Segment Reporting Information [Line Items]
All assets of the business segments									$ 228
Solar energy projects	40	70
Purchase price of solar energy development company	$ 51	$ 288	$ 2	$ 12	$ 4	$ 200	$ 340	$ 39
Number of companies purchased	2