Outstanding Balances of Short-Term Debt by Type of Collateral Securing Debt (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013 Contract	Dec. 31, 2012 Contract
Short-term Debt [Line Items]
Number of Facilities	11	11
Outstanding	$ 1,445,961	$ 551,918
Residential Loans
Short-term Debt [Line Items]
Number of Facilities	4	4
Outstanding	1,082,726	179,982
Limit	1,350,000	800,000
Residential Loans | Minimum
Short-term Debt [Line Items]
Maturity	2013-07	2013-01
Residential Loans | Maximum
Short-term Debt [Line Items]
Maturity	2014-01	2013-11
Real Estate Securities
Short-term Debt [Line Items]
Number of Facilities	7	7
Outstanding	$ 363,235	$ 371,936
Real Estate Securities | Minimum
Short-term Debt [Line Items]
Maturity	2013-07	2013-01
Real Estate Securities | Maximum
Short-term Debt [Line Items]
Maturity	2013-08	2013-03