Short-Term Debt by Weighted Average Interest Rates and Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Amount Borrowed	$ 838,299	$ 551,918
Weighted Average Interest Rate	1.68%	1.81%
Weighted Average Days Until Maturity	100 days	84 days
Residential Loans
Short-term Debt [Line Items]
Amount Borrowed	461,584	179,982
Weighted Average Interest Rate	1.74%	1.76%
Weighted Average Days Until Maturity	157 days	212 days
Commercial Loans
Short-term Debt [Line Items]
Amount Borrowed	5,514
Weighted Average Interest Rate	2.44%
Weighted Average Days Until Maturity	208 days
Real Estate Securities
Short-term Debt [Line Items]
Amount Borrowed	$ 371,201	$ 371,936
Weighted Average Interest Rate	1.59%	1.83%
Weighted Average Days Until Maturity	27 days	23 days