Significant Accounting Policies (Details) (USD $)																	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	1 Months Ended
	Sep. 30, 2012 item	Dec. 31, 2011 item	Sep. 30, 2012 Maximum [Member]	Dec. 31, 2011 Maximum [Member]	Sep. 30, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Sep. 30, 2012 Weighted Average [Member]	Dec. 31, 2011 Weighted Average [Member]	Sep. 30, 2012 TBA - Fixed Rate Agency Securities [Member]		Dec. 31, 2011 TBA - Fixed Rate Agency Securities [Member]		Sep. 30, 2012 TBA - Fixed Rate Agency Securities Sold Short [Member]		Dec. 31, 2011 TBA - Fixed Rate Agency Securities Sold Short [Member]		Sep. 30, 2012 Total Return Swaps [Member]	Dec. 31, 2011 Total Return Swaps [Member]	Sep. 30, 2012 Interest Rate Swaps [Member]	Dec. 31, 2011 Interest Rate Swaps [Member]	Sep. 30, 2012 Credit Default Swaps [Member]	Dec. 31, 2011 Credit Default Swaps [Member]	Sep. 30, 2012 Futures [Member]	Dec. 31, 2011 Futures [Member]	Sep. 30, 2012 Long-Term Incentive Plan Units [Member] Non-Manager [Member]	Aug. 31, 2010 Long-Term Incentive Plan Units [Member] Non-Employees [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount																	100.00%	354.00%	110.00%	111.00%	90.00%	92.00%	69.00%	65.00%
Reverse repurchase agreements remaining terms, days			180 days	180 days	2 days	3 days	68 days	33 days
Reverse repurchase agreements interest rate			2.80%	2.56%	0.31%	0.08%	1.07%	0.82%
Investments, Fair Value	$ 1,165,644,000	$ 1,212,483,000							$ 33,723,000	[1],[2],[3]	$ 32,033,000	[1],[4],[5]
Percentage of open reserve repurchase agreements	64.00%	73.00%
Fair value of asset subject to resecuritization	2,400,000
Number of counterparties	3	4
Receivable for securities sold	494,842,000	533,708,000							441,800,000		443,700,000
Fair value	456,824,000	462,394,000											443,191,000	[1],[6]	446,707,000	[1],[7]
Payable for securities purchased	178,453,000	127,517,000											33,700,000		32,500,000
Net short position, TBA													$ 409,500,000		$ 414,700,000
LTIP vesting period (years)																										3 years
Vesting period (years)																									1 year

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

[2] At September 30, 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 104.58%, 25.88%, and 0.67% of shareholders' equity, respectively.

[3] 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". Percentage of Shareholders' Rating Description Equity Unrated but Agency-Guaranteed 131.13 % Aaa/AAA/AAA 0.02 % Aa/AA/AA 0.45 % A/A/A 1.85 % Baa/BBB/BBB 8.25 % Ba/BB/BB or below 90.44 % Unrated 1.62 %

[4] At December 31, 2011, 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 142.04%, 63.62%, and 0.41% of shareholders' 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." Percentage of Rating Description Shareholders' Equity U.S. Treasury Securities 2.72 % Unrated but Agency-Guaranteed 206.07 % Aaa/AAA/AAA 1.25 % Aa/AA/AA 1.88 % A/A/A 5.44 % Baa/BBB/BBB 3.46 % Ba/BB/BB or below 103.36 % Unrated 2.71 %

[6] At September 30, 2012, the Company's short investments guaranteed by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation represented 75.14% and 13.74% of shareholders' equity, respectively.

[7] At December 31, 2011, the Company's short investments guaranteed by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, and the Government National Mortgage Association represented 93.03%, 19.18%, and 8.22% of shareholders' equity, respectively.