Significant Accounting Policies (Details) (USD $)			3 Months Ended	1 Months Ended									3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013 counterparties	Dec. 31, 2012 counterparties	Mar. 31, 2013 Long-Term Incentive Plan Units [Member] Non-Manager [Member]	Aug. 31, 2010 Long-Term Incentive Plan Units [Member] Non-Employees [Member]	Mar. 31, 2013 TBA - Fixed Rate Agency Securities [Member]		Dec. 31, 2012 TBA - Fixed Rate Agency Securities [Member]		Mar. 31, 2013 TBA - Fixed Rate Agency Securities Sold Short [Member]		Dec. 31, 2012 TBA - Fixed Rate Agency Securities Sold Short [Member]		Mar. 31, 2013 Maximum [Member]	Dec. 31, 2012 Maximum [Member]	Mar. 31, 2013 Minimum [Member]	Dec. 31, 2012 Minimum [Member]	Mar. 31, 2013 Weighted Average [Member]	Dec. 31, 2012 Weighted Average [Member]	Mar. 31, 2013 Interest Rate Swaps [Member]	Dec. 31, 2012 Interest Rate Swaps [Member]	Mar. 31, 2013 Total Return Swaps [Member]	Dec. 31, 2012 Total Return Swaps [Member]	Mar. 31, 2013 Credit Default Swaps [Member]	Dec. 31, 2012 Credit Default Swaps [Member]	Mar. 31, 2013 Futures [Member]	Dec. 31, 2012 Futures [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount																			135.00%	92.00%	91.00%	87.00%	95.00%	78.00%	73.00%	57.00%
Reverse repurchase agreements remaining terms													180 days	180 days	1 day	10 days	81 days	57 days
Reverse repurchase agreements interest rate													2.45%	2.31%	0.36%	0.37%	0.82%	0.77%
Percentage of open reserve repurchase agreements	56.00%	67.00%
Number of counterparties	2	2
Fair value of asset subject to resecuritization	$ 2,500,000
Fair Value	1,535,924,000	1,375,116,000			89,969,000	[1],[2],[3]	43,610,000	[1],[4],[5]
Receivable for securities sold	609,239,000	626,919,000			606,300,000		608,500,000
Investments sold short at fair value-	(649,756,000)	(622,301,000)							(607,285,000)	[1],[6]	(608,720,000)	[1],[7]
Payable for securities purchased	132,750,000	57,333,000							89,900,000		43,800,000
Net short position, TBA									$ (517,300,000)		$ (565,100,000)
Vesting period			1 year
LTIP vesting period				3 years

[1] See Note 2 and Note 3 in Notes to Consolidated Financial Statements.

[2] At March 31, 2013, the Company's long investments guaranteed by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, and the Government National Mortgage Association represented 116.33%, 51.87%, and 14.47% of equity, respectively.

[3] The table below shows the ratings on the Company's long investments from Moody's, Standard and Poor's, or Fitch, as well as the Company's long investments that were unrated but affiliated with Fannie Mae, Freddie Mac, or Ginnie Mae. Ratings tend to be a lagging credit indicator; as a result, the credit quality of the Company's long investment holdings may be lower than the credit quality implied based on the ratings listed below. In situations where an investment has a split rating, the lowest provided rating is used. The ratings descriptions include ratings qualified with a "+," "-," "1," "2," or "3."Rating Description Percent ofEquityUnrated but Agency-Guaranteed 182.67%Aaa/AAA/AAA 0.01%Aa/AA/AA 0.00%A/A/A 1.29%Baa/BBB/BBB 2.41%Ba/BB/BB or below 104.70%Unrated 3.95%

[4] At December 31, 2012, the Company’s long investments guaranteed by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, and the Government National Mortgage Association represented 119.78%, 40.49%, and 1.25% of equity, respectively.

[5] The table below shows the Company's long investment ratings from Moody's, Standard and Poor's, or Fitch, as well as the Company's long investments that were unrated but affiliated with Fannie Mae, Freddie Mac, or Ginnie Mae. Ratings tend to be a lagging credit indicator; as a result, the credit quality of the Company's long investment holdings may be lower than the credit quality implied based on the ratings listed below. In situations where an investment has a split rating, the lowest provided rating is used. The ratings descriptions include ratings qualified with a "+," "-," "1," "2," or "3."Rating Description Percent ofEquityUnrated but Agency-Guaranteed 161.52%Aaa/AAA/AAA 0.01%Aa/AA/AA 0.26%A/A/A 1.02%Baa/BBB/BBB 3.70%Ba/BB/BB or below 102.40%Unrated 2.66%

[6] At March 31, 2013, the Company's short investments guaranteed by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation represented 100.31% and 16.34% of equity, respectively.

[7] At December 31, 2012, the Company’s short investments guaranteed by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation represented 105.27% and 14.95% of equity, respectively.