Statements of Income (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest income:
Interest income on mortgage-backed securities	$ 131,381	$ 131,940	$ 128,720	$ 112,759	$ 114,465	$ 110,125	$ 113,188	$ 86,935	$ 504,800	$ 424,713	$ 263,751
Interest income on short-term cash investments	347	387	441	333	356	380	331	340	1,508	1,407	1,265
Interest income	131,728	132,327	129,161	113,092	114,821	110,505	113,519	87,275	506,308	426,120	265,016
Interest expense	57,019	52,767	46,169	41,109	42,299	40,259	35,910	26,194	197,064	144,662	95,923
Net interest income	74,709	79,560	82,992	71,983	72,522	70,246	77,609	61,081	309,244	281,458	169,093
Operating expenses:
Management fee	4,709	4,557	4,312	3,842	3,592	3,572	3,531	3,092	17,420	13,787	9,205
Share based compensation	619	447	425	429	415	334	194	207	1,920	1,150	1,432
General and administrative	1,737	1,040	1,316	913	731	688	746	559	5,006	2,724	2,507
Total operating expenses	7,065	6,044	6,053	5,184	4,738	4,594	4,471	3,858	24,346	17,661	13,144
Other income/(expense):
Net gain on sale of mortgage-backed securities	39,103	10,534	12,205	2,505	2,841	13,330	4,405		64,347	20,576	13,551
Net income	106,747	84,050	89,144	69,304	70,625	78,982	77,543	57,223	349,245	284,373	169,500
Dividends on preferred stock	5,481	2,070	0	0					7,551	0	0
Net income available to common shareholders	$ 101,266	$ 81,980	$ 89,144	$ 69,304	$ 70,625	$ 78,982	$ 77,543	$ 57,223	$ 341,694	$ 284,373	$ 169,500
Earnings per share - common stock, basic	$ 1.02	$ 0.83	$ 0.91	$ 0.89	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 3.67	$ 3.97	$ 4.30
Earnings per share - common stock, diluted	$ 1.02	$ 0.83	$ 0.91	$ 0.89	$ 0.92	$ 1.04	$ 1.04	$ 0.96	$ 3.67	$ 3.97	$ 4.30
Weighted average common shares outstanding, basic									93,185,520	71,708,058	39,454,362
Weighted average common shares outstanding, diluted									93,185,520	71,708,058	39,454,362