Transactions with Affiliates - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended
	Sep. 30, 2008	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2012 Minimum [Member] Y	Dec. 31, 2012 Maximum [Member] Y	Sep. 30, 2011 Subsidiaries [Member]	Dec. 31, 2012 Fees and revenue sharing arrangement [Member]	Dec. 31, 2011 Fees and revenue sharing arrangement [Member]	Dec. 31, 2010 Fees and revenue sharing arrangement [Member]
Related Party Transaction [Line Items]
Unsecured revolving credit facility with BTMU	$ 500
Expires of revolving Credit Facility	Jul. 01, 2015
Unsecured revolving credit facility years	3 years
Risk-weighted assets		2.00%
Project Related expenditure		39	42		29
Increase in assets								990
Short term borrowing		1,363	3,683					38
Capital contribution from BTMU			783	[1]				783
Derivative contract notional Balance		415	462
Derivative net unrealized losses		10	10
Income									21	18	16
Expenses									$ 18	$ 12	$ 10
Lease transaction term						10	34

[1] For additional information on the capital contribution, refer to Note 22 to these consolidated financial statements.