Mortgage Loans Narratives (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Mortgage Loans on Real Estate [Abstract]
Minimum past due months of single-family nonaccrual loans	2 months		2 months		2 months
Average term extension of a single-family modified loan	154 months	119 months	151 months	124 months
Average interest rate reduction of a single-family modified loan	1.70%	2.34%	1.76%	2.30%