Mortgage Loans Narratives (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Abstract]
Carrying value of loans redesignated from HFI to HFS	$ 33	$ 561
Average term extension of a single-family modified loan	131 months	90 months
Average interest rate reduction of a single-family modified loan	2.15%	2.95%