Unsecured Borrowings (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 years		Dec. 31, 2010 years	Dec. 31, 2009
Debt Instrument [Line Items]
Junior subordinated notes maturity term (years)	30
Remaining issue costs	$ 2,400,000
Term of LIBOR interest rate agreement (years)	20		20
Notional amount of swap agreement			100,000,000
Average fixed rates of interest rate swap agreements			4.09%
Unsecured borrowings, average rate	8.49%	[1]	8.49%	8.49%
Interest paid on borrowings outstanding	8,600,000
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
For Initial Ten Years [Member]
Debt Instrument [Line Items]
Junior subordinated notes interest rate, minimum	8.19%
Junior subordinated notes interest rate, maximum	8.685%
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	October 2035
Unsecured borrowings, average rate	8.31%	[1]	8.31%
Capstead Mortgage Trust II [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	December 2035
Junior subordinated notes, earliest redemption option exercise date	Dec. 15, 2015
Unsecured borrowings, average rate	8.46%	[1]	8.46%
Capstead Mortgage Trust III [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	September 2036
Junior subordinated notes, earliest redemption option exercise date	Sep. 15, 2016
Unsecured borrowings, average rate	8.78%	[1]	8.78%
Maximum [Member]
Debt Instrument [Line Items]
Interest rate spread over three-month LIBOR	3.50%
Minimum [Member]
Debt Instrument [Line Items]
Interest rate spread over three-month LIBOR	3.30%

[1] After considering cash flow hedges that coincide with the floating rate terms of these borrowings that begin in 2015 and 2016, 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.