Allowance for Credit Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Total recorded investment	$ 7,253,832		$ 7,575,877
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance, beginning of period	17,907	20,750
Charge-offs	(575)	(1,160)
Reversal of Loan Losses Expensed	(2,500)
Provision for credit losses		1,410
Balance, end of period	14,832	21,000
Conventional Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Credit Losses, Collectively Evaluated for Impairment	14,744		17,775
Allowance for Credit Losses, Individually Evaluated for Impairment	88		132
Recorded Investment, Collectively Evaluated for Impairment	6,313,305		6,570,856
Recorded Investment, Individually Evaluated for Impairment	6,219		5,249
Total recorded investment	$ 6,319,524		$ 6,576,105