Unsecured Borrowings (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Junior subordinated notes maturity term (in years)	30 years
Remaining issue costs	$ 2.3
Debt, Weighted Average Interest Rate	8.49%	[1]	8.49%	8.49%
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	2035
Capstead Mortgage Trust II [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	2035
Junior subordinated notes, earliest redemption option exercise date	Dec. 15, 2015
Capstead Mortgage Trust III [Member]
Debt Instrument [Line Items]
Junior subordinated notes, maturity period	2036
Junior subordinated notes, earliest redemption option exercise date	Sep. 15, 2016
Debt, Weighted Average Interest Rate	8.49%
Interest paid on outstanding borrowings	8.6
Special-Purpose Statutory Trusts [Member]
Debt Instrument [Line Items]
Issuance of common securities	3.1
Proceeds from private placement of preferred securities	$ 100

[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.