Short-Term Debt by Weighted Average Interest Rates and Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Amount Borrowed	$ 1,445,961	$ 551,918
Weighted Average Interest Rate	1.76%	1.81%
Weighted Average Days Until Maturity	95 days	84 days
Residential Loans
Short-term Debt [Line Items]
Amount Borrowed	1,082,726	179,982
Weighted Average Interest Rate	1.76%	1.76%
Weighted Average Days Until Maturity	120 days	212 days
Real Estate Securities
Short-term Debt [Line Items]
Amount Borrowed	$ 363,235	$ 371,936
Weighted Average Interest Rate	1.76%	1.83%
Weighted Average Days Until Maturity	22 days	23 days