Management Agreement and Related Party Transactions (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended									3 Months Ended								12 Months Ended			1 Months Ended		12 Months Ended
	Jul. 31, 2012	Mar. 31, 2012	Nov. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jan. 31, 2011	Dec. 31, 2010	Oct. 31, 2010	May 31, 2010	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2010 Private Placement [Member] External Management [Member]	Jul. 31, 2009 Private Placement [Member] External Management [Member]	Dec. 31, 2008 Private Placement [Member] External Management [Member]
Related Party Transaction [Line Items]
Management fees										$ 31	$ 32	$ 28	$ 22	$ 18	$ 16	$ 12	$ 8	$ 113	$ 55	$ 11
Expense Reimbursement to Manager																		9	7	3
Management Fee Payable										1				8				1	8
Related Party Transaction, Sale of Stock by Related Party, Shares																							5.0
Related Party Transaction, Sale of Stock, Price per Share																							$ 20.00
Proceeds from Issuance of Common Stock	$ 1,240	$ 2,063	$ 1,108	$ 1,369	$ 892	$ 719	$ 227	$ 328	$ 169									$ 3,601	$ 4,377	$ 1,055			$ 100
Related Party Transaction, Sale of Stock by Related Party, Shares																					2.5	2.5