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
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments	1.73%		2.14%
Other	0.11%		0.15%
Interest income	1.68%		2.08%
Repurchase arrangements and similar borrowings	0.58%		0.49%
Unsecured borrowings	8.49%	[1]	8.49%	8.49%	[1]
Interest expense	0.64%		0.56%
Net interest income (expense)	1.04%		1.52%
Residential mortgage investments	$ 58,468		$ 65,733
Other	112		150
Interest income	58,580		65,883
Repurchase arrangements and similar borrowings	(18,468)		(14,103)
Unsecured borrowings	(2,187)		(2,187)
Interest expense	(20,655)		(16,290)
Interest expense	20,655		16,290
Net interest income (expense)	37,925		49,593
Related Changes In Rate [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments	(13,557)	[2]
Other	(51)	[2]
Interest income	(13,608)	[2]
Repurchase arrangements and similar borrowings	2,783	[2]
Interest expense	2,783	[2]
Net interest income (expense)	(16,391)	[2]
Related Changes In Volume [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments	6,292	[2]
Other	14	[2]
Interest income	6,306	[2]
Repurchase arrangements and similar borrowings	1,582	[2]
Interest expense	1,582	[2]
Net interest income (expense)	4,724	[2]
Related Changes In Total [Member]
Component of Other Expense, Nonoperating [Line Items]
Residential mortgage investments	(7,265)	[2]
Other	(37)	[2]
Interest income	(7,302)	[2]
Repurchase arrangements and similar borrowings	4,365	[2]
Interest expense	4,365	[2]
Net interest income (expense)	$ (11,667)	[2]

[1] The indicated weighted average rates have been in effect since issuance. 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.