Debt Financing Total Debt Financing (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Debt financing	$ 194,267,900	$ 177,948,000
Categories of Investments, Marketable Securities, Trading Securities [Member]
Short-term Debt [Line Items]
Debt financing	98,310,000	83,960,000
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-2 [Member]
Short-term Debt [Line Items]
Debt financing	48,995,000	48,995,000
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2013	Jul. 01, 2013
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB Facility-1 [Member]
Short-term Debt [Line Items]
Debt financing	9,850,000	9,850,000
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2013	Jul. 01, 2013
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-5 [Member]
Short-term Debt [Line Items]
Debt financing	5,745,000
Available-for-sale Security, Maturity Date at a point in time	Dec. 01, 2013
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-6 [Member]
Short-term Debt [Line Items]
Debt financing	7,000,000
Available-for-sale Security, Maturity Date at a point in time	Feb. 01, 2013
Mortgage Backed Security Trust 1 [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	2,585,000	2,585,000
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013	Oct. 01, 2013
Mortgage Backed Security Trust 2 [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	4,090,000	4,090,000
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013	Oct. 01, 2013
Mortgage Backed Security Trust 3 [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	3,540,000	3,890,000
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013	Oct. 01, 2013
Mortgage Backed Security Trust 4 [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	5,960,000	5,960,000
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013	Oct. 01, 2013
Mortgage Backed Security Trust 5 [Member] [Member] | Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	$ 10,545,000	$ 8,590,000
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013	Oct. 01, 2013