Debt Financing Total Debt Financing (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Debt financing	$ 366,145,894	$ 345,359,000
Trading Securities [Member]
Short-term Debt [Line Items]
Debt financing	195,640,000	174,250,000
Trading Securities [Member] | TOB facility-2 [Member]
Short-term Debt [Line Items]
Debt financing	$ 43,985,000	$ 44,675,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	2.30%	2.20%
Available-for-sale Security, Maturity Date at a point in time	Dec. 01, 2015	Jun. 01, 2015
Trading Securities [Member] | TOB facility-9 [Member]
Short-term Debt [Line Items]
Debt financing	$ 15,000,000	$ 25,535,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.05%
Available-for-sale Security, Maturity Date at a point in time	Nov. 01, 2015	Jun. 01, 2015
Trading Securities [Member] | TOB Facility-12 [Member] [Member]
Short-term Debt [Line Items]
Debt financing	$ 17,250,000	$ 17,250,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.55%	4.55%
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2017	Jul. 01, 2017
Trading Securities [Member] | TOB Facility-10 [Member]
Short-term Debt [Line Items]
Debt financing	$ 21,850,000	$ 21,850,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.34%	4.34%
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2016	Oct. 01, 2016
Trading Securities [Member] | TOB Facility-11 [Member]
Short-term Debt [Line Items]
Debt financing	$ 34,940,000	$ 34,975,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.47%	4.47%
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2019	Jul. 01, 2019
Trading Securities [Member] | TOB facility-13 [Member]
Short-term Debt [Line Items]
Debt financing	$ 9,010,000	$ 9,010,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.05%	4.05%
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2017	Jul. 01, 2017
Trading Securities [Member] | TOB facility-14 [Member]
Short-term Debt [Line Items]
Debt financing	$ 8,375,000	$ 9,010,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	4.05%	4.05%
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2017	Jul. 01, 2017
Trading Securities [Member] | TOB facility-15 [Member]
Short-term Debt [Line Items]
Debt financing	$ 11,940,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.80%
Available-for-sale Security, Maturity Date at a point in time	Feb. 01, 2018
Trading Securities [Member] | TOB facility-16 [Member]
Short-term Debt [Line Items]
Debt financing	$ 8,735,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.80%
Available-for-sale Security, Maturity Date at a point in time	Feb. 01, 2018
Trading Securities [Member] | TOB facility-17 [Member]
Short-term Debt [Line Items]
Debt financing	$ 12,610,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	2.80%
Available-for-sale Security, Maturity Date at a point in time	Feb. 01, 2018
Mortgage Backed Security Trust 1 [Member] | Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	$ 2,585,000	$ 2,585,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	1.16%	1.12%
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2015	Apr. 01, 2015
Mortgage Backed Security Trust 2 [Member] | Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	$ 4,090,000	$ 4,090,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	1.16%	1.12%
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2015	Apr. 01, 2015
Mortgage Backed Security Trust 5 [Member] [Member] | Trading Securities [Member] | TOB facility-3 [Member]
Short-term Debt [Line Items]
Debt financing	$ 5,270,000	$ 5,270,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	1.16%	1.06%
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2015	Apr. 01, 2015
Mortgage Backed Security Trust 5 [Member] [Member] | Trading Securities [Member] | TOB facility-9 [Member]
Short-term Debt [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate		1.96%