Unsecured Borrowings (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2012 Year		Dec. 31, 2010 Year	Dec. 31, 2011	Jun. 30, 2011
Debt Instrument [Line Items]
Junior subordinated notes maturity term (in years)	30
Remaining issue costs	$ 2.4
Term of LIBOR interest rate agreement (in years)	20		20
Debt, Weighted Average Interest Rate	8.49%	[1]		0.37%	8.49%
Special-Purpose Statutory Trusts [Member]
Debt Instrument [Line Items]
Issuance of common securities	3.1
Proceeds from private placement of preferred securities	$ 100
For Initial Ten Years [Member]
Debt Instrument [Line Items]
Junior subordinated notes interest rate, minimum	8.19%
Junior subordinated notes interest rate, maximum	8.685%
Capstead Mortgage Trust I [Member]
Debt Instrument [Line Items]
Junior subordinated notes, initial term (in years)	10
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%
Minimum
Debt Instrument [Line Items]
Interest rate spread over three-month LIBOR	3.30%
Maximum
Debt Instrument [Line Items]
Interest rate spread over three-month LIBOR	3.50%

[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.