Unsecured Borrowings (Schedule Of Subordinated Note Balances And Related Weighted Average Interest Rate) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Y		Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Borrowings Outstanding	$ 103,095		$ 103,095
Average Rate	8.49%	[1]	8.49%	8.49%
Junior subordinated notes, remaining maturity	20
For Final 20 Years [Member]
Debt Instrument [Line Items]
Effective borrowing rate considering cash flow hedges	7.56%
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	36,083
Average Rate	8.31%	[1]
Capstead Mortgage Trust II [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	41,238
Average Rate	8.46%	[1]
Capstead Mortgage Trust III [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 25,774
Average Rate	8.78%	[1]

[1] 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.