Unsecured Borrowings (Schedule Of Subordinated Note Balances And Related Weighted Average Interest Rate) (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 Year		Dec. 31, 2011	Jun. 30, 2011
Debt Instrument [Line Items]
Borrowings Outstanding	$ 103,095		$ 103,095
Average Rate	8.49%	[1]	0.37%	8.49%
Junior subordinated notes, remaining maturity	20
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]
For Final 20 Years [Member]
Debt Instrument [Line Items]
Effective borrowing rate considering cash flow hedges	7.56%

[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.