Unsecured Borrowings (Schedule Of Subordinated Note Balances And Related Weighted Average Interest Rate) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Debt Instrument [Line Items]
Junior subordinated notes, Borrowings Outstanding	$ 103,095	$ 103,095
Junior subordinated notes, Average Rate	8.49%	8.49%	8.49%
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Junior subordinated notes, Borrowings Outstanding	36,083	36,083
Junior subordinated notes, Average Rate	8.31%	8.31%
Capstead Mortgage Trust II [Member]
Debt Instrument [Line Items]
Junior subordinated notes, Borrowings Outstanding	41,238	41,238
Junior subordinated notes, Average Rate	8.46%	8.46%
Capstead Mortgage Trust III [Member]
Debt Instrument [Line Items]
Junior subordinated notes, Borrowings Outstanding	$ 25,774	$ 25,774
Junior subordinated notes, Average Rate	8.78%	8.78%