Unsecured Borrowings (Narrative) (Details) (USD $)	6 Months Ended			6 Months Ended						6 Months Ended	12 Months Ended	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Mar. 31, 2008	Jun. 30, 2011 Maximum [Member]	Jun. 30, 2011 Minimum [Member]	Jun. 30, 2011 Capstead Mortgage Trust I [Member]	Jun. 30, 2011 Capstead Mortgage Trust II [Member]	Jun. 30, 2011 Capstead Mortgage Trust III [Member]	Dec. 31, 2010 Junior Subordinated Notes [Member]	Jun. 30, 2011 For Initial Ten Years [Member]	Dec. 31, 2010 For Final Twenty Years [Member]	Jun. 30, 2011 Special-purpose Statutory Trusts [Member]
Junior subordinated notes maturity term (years)	30
Issuance of common securities												$ 3,100,000
Proceeds from private placement of preferred securities												100,000,000
Remaining issue costs	2,500,000
Junior subordinated notes interest rate, minimum										8.19%
Junior subordinated notes interest rate, maximum										8.685%
Interest rate spread over three-month LIBOR				3.50%	3.30%
Junior subordinated notes, maturity period						October 2035	December 2035	September 2036
Junior subordinated notes, earliest redemption option exercise date						Oct. 30, 2010	Dec. 15, 2015	Sep. 15, 2016
Notional amount of swap agreement			$ 100,000,000						$ 100,000,000
Effective borrowing rate considering cash-flow hedges											7.56%
Average fixed rates of interest rate swap agreements		4.09%
Term of interest rate swap agreement (in years)		20