Net Interest Income Analysis (Summary Of Interest Income, Interest Expense And Weighted Average Interest Rates) (Detail) (USD $) In Thousands, 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
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments										$ 255,931		$ 243,077		$ 198,488
Other										698		301		1,290
Interest Income	61,166		63,617	65,963	65,883	63,981	62,949	63,194	53,254	256,629		243,378		199,778
Repurchase arrangements and similar borrowings										69,101		57,328		47,502
Unsecured borrowings										(8,747)		(8,747)		(8,747)
Other												5		2
Interest expense	22,859		20,061	18,638	16,290	17,743	17,930	15,894	14,513	77,848		66,080		56,251
Net interest income (expense)	38,307		43,556	47,325	49,593	46,238	45,019	47,300	38,741	178,781		177,298		143,527
Residential mortgage investments										1.94%		2.24%		2.59%
Other										0.16%		0.12%		0.49%
Interest Income										1.88%		2.20%		2.52%
Repurchase arrangements and similar borrowings										0.56%		0.56%		0.66%
Unsecured borrowings	8.49%	[1]				8.49%				8.49%	[1]	8.49%		8.49%
Other										0.07%		0.14%		0.19%
Interest expense										0.62%		0.64%		0.78%
Net interest income (expense)										1.26%		1.56%		1.74%
Related Changes In Rate [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments										(35,212)	[2]	(29,727)	[2]
Other										104	[2]	(889)	[2]
Interest Income										(35,108)	[2]	(30,616)	[2]
Repurchase arrangements and similar borrowings												(7,839)	[2]
Other										(2)	[2]
Interest expense										(2)	[2]	(7,839)	[2]
Net interest income (expense)										(35,106)	[2]	(22,777)	[2]
Related Changes In Volume [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments										48,066	[2]	74,317	[2]
Other										293	[2]	(101)	[2]
Interest Income										48,359	[2]	74,216	[2]
Repurchase arrangements and similar borrowings										11,773	[2]	17,665	[2]
Other										(3)	[2]	3	[2]
Interest expense										11,770	[2]	17,668	[2]
Net interest income (expense)										36,589	[2]	56,548	[2]
Related Changes In Total [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments										12,854	[2]	44,590	[2]
Other										397	[2]	(990)	[2]
Interest Income										13,251	[2]	43,600	[2]
Repurchase arrangements and similar borrowings										11,773	[2]	9,826	[2]
Other										(5)	[2]	3	[2]
Interest expense										11,768	[2]	9,829	[2]
Net interest income (expense)										$ 1,483	[2]	$ 33,771	[2]

[1] After considering cash flow hedges that coincide with the floating rate terms of these borrowings that begin in October and December 2015 for the Capstead Mortgage Trust I and II notes and September 2016 for the Capstead Mortgage Trust III notes, the effective borrowing rate during the final 20 years of these borrowings will average 7.56%, subject to certain adjustments for the effects of measured hedge ineffectiveness, if any.

[2] The change in interest income and interest expense due to both rate and volume has been allocated to rate and volume changes in proportion to the relationship of the absolute dollar amounts of the change in each.