Significant Accounting Policies (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Significant Accounting Policies [Line Items]
Investments, Fair Value	1,131,242,000		1,212,483,000
Percentage of open reserve repurchase agreements	61.00%		73.00%
Fair value of asset subject to resecuritization	2,300,000
Number of counterparties	3		4
Receivable for securities sold	611,866,000		533,708,000
Fair value	494,524,000		462,394,000
Payable for securities purchased	83,300,000		127,517,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Reverse repurchase agreements remaining terms, days	180		180
Reverse repurchase agreements interest rate	2.75%		2.56%
Minimum [Member]
Significant Accounting Policies [Line Items]
Reverse repurchase agreements remaining terms, days	2		3
Reverse repurchase agreements interest rate	0.05%		0.08%
Weighted Average [Member]
Significant Accounting Policies [Line Items]
Reverse repurchase agreements remaining terms, days	45		33
Reverse repurchase agreements interest rate	0.86%		0.82%
TBA - Fixed Rate Agency Securities [Member]
Significant Accounting Policies [Line Items]
Investments, Fair Value	57,739,000	[1],[2],[3]	32,033,000	[1],[4],[5]
Receivable for securities sold	457,100,000		443,700,000
Net short position, TBA	400,300,000		414,700,000
TBA - Fixed Rate Agency Securities Sold Short [Member]
Significant Accounting Policies [Line Items]
Fair value	458,028,000	[1],[6]	446,707,000	[1],[7]
Payable for securities purchased	58,100,000		32,500,000
Credit Default Swaps [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount	95.00%		92.00%
Interest Rate Swaps [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount	97.00%		111.00%
Total Return Swaps [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount	102.00%		354.00%
Futures [Member]
Significant Accounting Policies [Line Items]
Percentage of average monthly notional amount	80.00%		65.00%

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

[2] At June 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 147.05%, 31.99%, and 0.10% 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 U.S. Treasury Securities 1.27 % Unrated but Agency-Guaranteed 179.14 % Aaa/AAA/AAA 0.33 % Aa/AA/AA 0.57 % A/A/A 2.39 % Baa/BBB/BBB 2.82 % Ba/BB/BB or below 98.49 % Unrated 1.18 %

[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 June 30, 2012, the Company's short investments guaranteed by the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation represented 111.11% and 4.77% 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.