Consolidated Condensed Schedule Of Investments (Investments Sold Short) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Investments Sold, Not yet Purchased [Line Items]
Investments sold short at fair value-	$ (649,756)		$ (622,301)
Proceeds from investments sold short	(648,101)		(621,048)
TBA - Fixed Rate Agency Securities Sold Short [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold short at fair value-	(607,285)	[1],[2]	(608,720)	[1],[3]
Proceeds from investments sold short	(606,162)	[1],[2]	(607,967)	[1],[3]
Investments sold not yet purchased as a percentage of net assets	(116.65%)	[1],[2],[4]	(120.22%)	[1],[3],[4]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 3.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(134,048)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	3.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(141,541)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 4.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(79,730)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	4.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(85,894)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 3.50% 2013-05 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(78,200)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	3.50%	[1],[2]
Investments maturity date	2013-05	[1],[2]
Investments sold short at fair value-	(82,373)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 3.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(66,180)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	3.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(68,256)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal Home Loan Mortgage Corporation (30 Year) 3.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(62,900)	[1],[2]
Description	Federal Home Loan Mortgage Corporation (30 Year)	[1],[2]
Rate	3.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(66,202)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 5.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(55,400)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	5.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(60,020)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 3.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(17,910)	[1],[2]
Description	Federal National Mortgage Association (15 Year)	[1],[2]
Rate	3.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(18,839)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | 16,500 Federal Home Loan Mortgage Corporation (30 Year) 5.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(16,500)	[1],[2]
Description	Federal Home Loan Mortgage Corporation (30 Year)	[1],[2]
Rate	5.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(17,753)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 4.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(15,460)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	4.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(16,482)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 4.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(13,534)	[1],[2]
Description	Federal National Mortgage Association (15 Year)	[1],[2]
Rate	4.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(14,481)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | 11,000 Federal National Mortgage Association (15 Year) 4.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(11,000)	[1],[2]
Description	Federal National Mortgage Association (15 Year)	[1],[2]
Rate	4.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(11,828)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 5.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(7,500)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	5.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(8,180)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 4.00% 2013-05 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(6,600)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	4.00%	[1],[2]
Investments maturity date	2013-05	[1],[2]
Investments sold short at fair value-	(7,028)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 3.50% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(4,320)	[1],[2]
Description	Federal National Mortgage Association (15 Year)	[1],[2]
Rate	3.50%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(4,581)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 6.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(2,500)	[1],[2]
Description	Federal National Mortgage Association (30 Year)	[1],[2]
Rate	6.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(2,738)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Other Federal Home Loan Mortgage Corporation (30 Year) 6.00% 2013-04 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(1,000)	[1],[2]
Description	Other Federal Home Loan Mortgage Corporation (30 Year)	[1],[2]
Rate	6.00%	[1],[2]
Investments maturity date	2013-04	[1],[2]
Investments sold short at fair value-	(1,089)	[1],[2]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold not yet purchased as a percentage of net assets	(100.31%)		(105.27%)
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 3.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(198,168)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			3.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(211,343)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 5.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(87,500)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			5.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(94,794)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 4.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(77,730)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			4.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(83,982)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal Home Loan Mortgage Corporation (30 Year) 3.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(53,400)	[1],[3]
Description			Federal Home Loan Mortgage Corporation (30 Year)	[1],[3]
Rate			3.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(56,804)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 3.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(34,510)	[1],[3]
Description			Federal National Mortgage Association (15 Year)	[1],[3]
Rate			3.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(36,430)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 3.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(31,600)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			3.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(33,128)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal Home Loan Mortgage Corporation (30 Year) 5.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(16,500)	[1],[3]
Description			Federal Home Loan Mortgage Corporation (30 Year)	[1],[3]
Rate			5.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(17,776)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 3.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(16,450)	[1],[3]
Description			Federal National Mortgage Association (15 Year)	[1],[3]
Rate			3.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(17,460)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 4.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(15,718)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			4.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(16,852)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 4.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(14,104)	[1],[3]
Description			Federal National Mortgage Association (15 Year)	[1],[3]
Rate			4.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(15,096)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (15 Year) 4.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(11,000)	[1],[3]
Description			Federal National Mortgage Association (15 Year)	[1],[3]
Rate			4.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(11,833)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 5.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(7,500)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			5.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(8,148)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal National Mortgage Association (30 Year) 6.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(2,500)	[1],[3]
Description			Federal National Mortgage Association (30 Year)	[1],[3]
Rate			6.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(2,731)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Other Federal National Mortgage Association (15 Year) 2.50% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(1,200)	[1],[3]
Description			Other Federal National Mortgage Association (15 Year)	[1],[3]
Rate			2.50%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(1,255)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Other Federal Home Loan Mortgage Corporation (30 Year) 6.00% 2013-01 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount			(1,000)	[1],[3]
Description			Other Federal Home Loan Mortgage Corporation (30 Year)	[1],[3]
Rate			6.00%	[1],[3]
Investments maturity date			2013-01	[1],[3]
Investments sold short at fair value-			(1,088)	[1],[3]
TBA - Fixed Rate Agency Securities Sold Short [Member] | Federal Home Loan Mortgage Corporation [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold not yet purchased as a percentage of net assets			(14.95%)
TBA - Fixed Rate Agency Securities Sold Short [Member] | Government National Mortgage Association [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold not yet purchased as a percentage of net assets	(16.34%)
U.S. Treasury Securities Sold Short [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold short at fair value-	(42,471)	[1]	(13,581)	[1]
Proceeds from investments sold short	(41,939)	[1]	(13,081)	[1]
Investments sold not yet purchased as a percentage of net assets	(8.16%)	[1],[4]	(2.68%)	[1],[4]
U.S. Treasury Securities Sold Short [Member] | U.S. Treasury Note 2.00% 2023-02 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(18,700)	[1]
Description	U.S. Treasury Note	[1]
Rate	2.00%	[1]
Investments maturity date	2023-02	[1]
Investments sold short at fair value-	(18,947)	[1]
U.S. Treasury Securities Sold Short [Member] | U.S. Treasury Note 1.75% 2016-05 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(13,000)	[1]	(13,000)	[1]
Description	U.S. Treasury Note	[1]	U.S. Treasury Note	[1]
Rate	1.75%	[1]	1.75%	[1]
Investments maturity date	2016-05	[1]	2016-05	[1]
Investments sold short at fair value-	(13,554)	[1]	(13,581)	[1]
U.S. Treasury Securities Sold Short [Member] | U.S. Treasury Note 1.13% 2019-02 [Member]
Investments Sold, Not yet Purchased [Line Items]
Current Principal/Notional Amount	(10,000)	[1]
Description	U.S. Treasury Note	[1]
Rate	1.13%	[1]
Investments maturity date	2019-02	[1]
Investments sold short at fair value-	(9,970)	[1]
Securities Sold Short - Securities Sold, Not Yet Purchased [Member]
Investments Sold, Not yet Purchased [Line Items]
Investments sold short at fair value-	(649,756)	[1]	(622,301)	[1]
Proceeds from investments sold short	$ (648,101)	[1]	$ (621,048)	[1]
Investments sold not yet purchased as a percentage of net assets	(124.81%)	[1],[4]	(122.90%)	[1],[4]

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

[2] 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.

[3] 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.

[4] Classification percentages are based on Total Equity.