Summarized Quarterly Results (Detail) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest income:
Interest-Agency MBS	$ 19,964,000	$ 18,494,000	$ 10,690,000	$ 20,765,000	$ 22,881,000	$ 24,572,000	$ 27,464,000	$ 30,588,000	$ 69,912,000	$ 105,504,000	$ 155,486,000
Interest-Non-Agency MBS	8,667,000	8,766,000	9,137,000	9,281,000	9,021,000	8,078,000	6,385,000	3,647,000	35,852,000	27,131,000	1,039,000
Interest-residential mortgage loans	7,854,000	8,359,000	9,111,000	9,313,000	7,510,000	4,120,000	1,188,000	0	34,637,000	12,819,000	0
Other interest income	17,000	11,000	12,000	12,000	13,000	11,000	10,000	10,000	51,000	44,000	42,000
Interest and Dividend Income, Operating, Total	36,502,000	35,630,000	28,950,000	39,371,000	39,425,000	36,781,000	35,047,000	34,245,000	140,452,000	145,498,000	156,567,000
Interest expense:
Interest expense on repurchase agreements	9,532,000	8,615,000	8,960,000	9,398,000	8,437,000	8,167,000	7,400,000	6,689,000	36,505,000	30,694,000	78,643,000
Interest expense on asset-backed securities	7,548,000	7,918,000	8,414,000	8,599,000	6,841,000	3,729,000	1,075,000	0	32,480,000	11,645,000	0
Interest expense on junior subordinated notes	375,000	360,000	354,000	346,000	325,000	323,000	319,000	315,000	1,435,000	1,282,000	1,264,000
Interest Expense, Total	17,455,000	16,893,000	17,728,000	18,343,000	15,603,000	12,219,000	8,794,000	7,004,000	70,420,000	43,621,000	79,907,000
Net interest income	19,047,000	18,737,000	11,222,000	21,028,000	23,822,000	24,562,000	26,253,000	27,241,000	70,032,000	101,877,000	76,660,000
Provision for loan losses	0	0	0	0	63,000	70,000	70,000	0	0	203,000	0
Net interest income after provision for loan losses	19,047,000	18,737,000	11,222,000	21,028,000	23,759,000	24,492,000	26,183,000	27,241,000	70,032,000	101,674,000	76,660,000
Operating expenses	(3,511,000)	(3,436,000)	(3,660,000)	(3,612,000)	(3,427,000)	(3,523,000)	(3,415,000)	(3,615,000)	(14,219,000)	(13,980,000)	(17,754,000)
Other income (loss):
Income-rental properties	425,000	424,000	428,000	410,000	416,000	399,000	409,000	370,000	1,688,000	1,594,000	589,000
(Loss) gain on sale of Agency MBS	(39,000)	1,206,000	0	(3,239,000)	(7,508,000)	0	0	0
Unrealized gain (loss) on Agency MBS held as trading investments	(14,925,000)	1,148,000	0	0					(13,800,000)	0
Gain on sale of Non-Agency MBS	137,000	0	0	0	1,000	0	(73,000)	(3,000)
Impairment charge on Non-Agency MBS	(1,548,000)	0	0	0					(1,548,000)	0	0
Gain on sales of residential mortgage loans held-for-investment	0	716,000	33,000	0					749,000	0	0
(Loss) gain on interest rate swaps, net	30,916,000	8,141,000	(16,089,000)	(50,219,000)	18,131,000	(50,965,000)	5,075,000	(46,488,000)	(27,250,000)	(74,247,000)	(36,676,000)
Gain (loss) on derivatives-TBA Agency MBS, net	(15,362,000)	3,412,000	8,061,000	15,354,000	(2,974,000)	10,345,000	(6,573,000)	8,525,000
Gain (loss) on derivatives-Eurodollar Futures Contracts	672,000	(2,060,000)	(1,357,000)	22,000	4,535,000	(2,569,000)	(1,732,000)	(2,338,000)
Recovery on Non-Agency MBS	9,000	1,000	1,000	1,000	13,000	7,000	4,000	1,000	12,000	26,000	120,000
Total other (loss)	285,000	12,988,000	(8,923,000)	(37,671,000)	12,614,000	(42,783,000)	(2,890,000)	(39,933,000)	(33,320,000)	(72,990,000)	(30,287,000)
Net income	15,821,000	28,289,000	(1,361,000)	(20,255,000)	32,946,000	(21,814,000)	19,878,000	(16,307,000)	22,493,000	14,704,000	28,619,000
Dividend on Series A Cumulative Preferred Stock	(1,035,000)	(1,035,000)	(1,035,000)	(1,035,000)	(1,035,000)	(1,035,000)	(1,035,000)	(1,035,000)	(4,139,000)	(4,139,000)	(4,139,000)
Dividend on Series B Cumulative Convertible Preferred Stock	(394,000)	(394,000)	(394,000)	(394,000)	(394,000)	(394,000)	(394,000)	(394,000)	(1,577,000)	(1,577,000)	(1,577,000)
Dividend on Series C Cumulative Redeemable Preferred Stock	(231,000)	(222,000)	(207,000)	(207,000)	(207,000)	(203,000)	(200,000)	(111,000)	(867,000)	(721,000)	0
Net income to common stockholders	$ 14,161,000	$ 26,638,000	$ (2,997,000)	$ (21,891,000)	$ 31,310,000	$ (23,446,000)	$ 18,249,000	$ (17,847,000)	$ 15,910,000	$ 8,267,000	$ 22,903,000
Basic earnings per common share	$ 0.15	$ 0.28	$ (0.03)	$ (0.22)	$ 0.31	$ (0.23)	$ 0.18	$ (0.17)	$ 0.17	$ 0.08	$ 0.18
Diluted earnings per common share	$ 0.14	$ 0.27	$ (0.03)	$ (0.22)	$ 0.30	$ (0.23)	$ 0.17	$ (0.17)	$ 0.17	$ 0.08	$ 0.18
Basic weighted average number of shares outstanding	95,706	95,881	96,355	97,704	99,857	102,431	104,225	107,228	96,408	103,412	123,949
Diluted weighted average number of shares outstanding	100,485	100,590	96,355	97,704	104,294	106,780	108,530	111,472	101,068	107,751	128,057