Short-Term Debt by Weighted Average Interest Rates and Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Amount Borrowed	$ 440,965	$ 428,056
Weighted Average Interest Rate	1.92%	1.92%
Weighted Average Days Until Maturity	105 days	240 days
Residential Loans
Short-term Debt [Line Items]
Amount Borrowed	136,861	307,149
Weighted Average Interest Rate	1.84%	1.93%
Weighted Average Days Until Maturity	238 days	322 days
Real Estate Securities
Short-term Debt [Line Items]
Amount Borrowed	$ 304,104	$ 120,907
Weighted Average Interest Rate	1.95%	1.88%
Weighted Average Days Until Maturity	46 days	31 days