Summarized Quarterly Financial Results (Quarterly Results Of Operations) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Interest income:
Interest income on mortgage-backed securities	$ 114,465	$ 110,125	$ 113,188	$ 86,935	$ 67,668	$ 63,701	$ 63,441	$ 68,941	$ 424,713	$ 263,751	$ 282,344
Interest income on short-term cash investments	356	380	331	340	284	323	298	277	1,407	1,265	771
Interest income	114,821	110,505	113,519	87,275	67,952	64,024	63,739	69,218	426,120	265,016	283,115
Interest expense	42,299	40,259	35,910	26,194	24,740	24,066	23,677	23,440	144,662	95,923	96,267
Net interest income	72,522	70,246	77,609	61,081	43,212	39,958	40,062	45,778	281,458	169,093	186,848
Net gain on sale of mortgage-backed securities	2,841	13,330	4,405		5,783	6,723		1,044	20,576	13,551
Management fee	3,592	3,572	3,531	3,092	2,505	2,321	2,196	2,183	13,787	9,205	8,677
Share based compensation	415	334	194	207	279	463	347	342	1,150	1,432	1,296
General and administrative	731	688	746	559	572	665	707	563	2,724	2,507	2,475
Total operating expenses	4,738	4,594	4,471	3,858	3,356	3,449	3,250	3,088	17,661	13,144	12,448
Net income	$ 70,625	$ 78,982	$ 77,543	$ 57,223	$ 45,639	$ 43,232	$ 36,812	$ 43,734	$ 284,373	$ 169,500	$ 174,400
Earnings per share - common stock, basic	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 0.99	$ 1.12	$ 1.01	$ 1.21
Earnings per share - common stock, diluted	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 0.99	$ 1.11	$ 1.01	$ 1.21
Dividends per share	$ 0.90	$ 1.00	$ 1.00	$ 1.00	$ 1.00	$ 1.10	$ 1.10	$ 1.20
Weighted average shares outstanding	76,606,611	75,743,002	74,807,174	59,442,488	46,100,493	38,765,078	36,609,290	36,241,823	71,708,058	39,454,362	36,195,840