UNSECURED BORROWINGS - Trusts (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Junior subordinated notes maturity term	30 years
Issuance of common securities	$ 3,100,000
Remaining issue costs	2,200,000		2,300,000
Junior subordinated notes borrowings outstanding	103,095,000		103,095,000
Average rate (in hundredths)	8.49%	[1]
Debt instrument remaining term	20 years
Effective borrowing rate considering cash flow hedges	7.56%
Special Purpose Statutory Trusts [Member]
Debt Instrument [Line Items]
Issuance of common securities	3,100,000
Proceeds from private placement of preferred securities	100,000,000
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Junior subordinated notes borrowings outstanding	36,083,000
Average rate (in hundredths)	8.31%	[1]
Junior subordinated notes, maturity period	Oct. 01, 2035
Capstead Mortgage Trust II [Member]
Debt Instrument [Line Items]
Junior subordinated notes borrowings outstanding	41,238,000
Average rate (in hundredths)	8.46%	[1]
Junior subordinated notes, maturity period	Dec. 01, 2035
Junior subordinated notes, earliest redemption option exercise date	Dec. 15, 2015
Capstead Mortgage Trust III [Member]
Debt Instrument [Line Items]
Junior subordinated notes borrowings outstanding	$ 25,774,000
Average rate (in hundredths)	8.78%	[1]
Junior subordinated notes, maturity period	Sep. 01, 2036
Junior subordinated notes, earliest redemption option exercise date	Sep. 15, 2016

[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.