Borrowings (Details) (USD $)	9 Months Ended			3 Months Ended			9 Months Ended			9 Months Ended			9 Months Ended
	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 PMT Sale and assignment agreement	Sep. 30, 2013 Loan Repo Facility	Sep. 30, 2012 Loan Repo Facility	Sep. 30, 2013 Loan Repo Facility	Sep. 30, 2012 Loan Repo Facility	Sep. 30, 2013 Loan Repo Facility Bank of America, N.A.	Sep. 30, 2013 Loan Repo Facility Credit Suisse First Boston Mortgage Capital LLC	Sep. 30, 2013 Loan Repo Facility Within 30 days	Sep. 30, 2013 Loan Repo Facility Over 30 to 90 days	Sep. 30, 2013 Loan Repo Facility Overnight cost-of funds rate item	Sep. 30, 2013 Loan Repo Facility LIBOR item
Mortgage loans sold under agreement to repurchase
Number of borrowing facilities	4
Number of borrowing facilities that provide for sales of mortgage loans under agreements to repurchase	3												1	2
Number of secured note payable borrowing facilities	1
Period end:
Balance	$ 387,883,000		$ 393,534,000		$ 387,883,000	$ 361,478,000	$ 387,883,000	$ 361,478,000			$ 423,000	$ 387,460,000
Unused amount					612,117,000	88,522,000	612,117,000	88,522,000
Weighted average interest rate (as a percent)					1.82%	2.38%	1.82%	2.38%
Fair value of mortgage loans securing agreements to repurchase	522,031,000		438,850,000		522,031,000	408,415,000	522,031,000	408,415,000
During the period:
Average balance of mortgage loans sold under agreements to repurchase					373,386,000	219,047,000	354,125,000	146,425,000
Weighted-average interest rate (as a percent)					1.89%	2.23%	2.02%	2.18%
Total interest expense					2,920,000	1,682,000	8,251,000	3,583,000
Maximum daily amount outstanding					588,494,000	361,588,000	623,523,000	361,588,000
Amortization of commitment fees excluded from calculation of Weighted average interest rate	3,714,000	1,261,000			1,100,000	423,000	2,800,000	1,100,000
Weighted-average maturity							2 months 18 days
Amount at risk									45,516,000	89,917,000
Deposit with loan repurchase agreement counterparties					1,500,000		1,500,000
Maturity period from any particular date of determination										364 days
Proceeds from sale of right to receive excess cash flow generated from the pool of mortgage loans				2,800,000
Carrying amount of the financing				$ 2,900,000