Debt Financing Total Debt Financing (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Debt financing	$ 177,948,000		$ 112,673,000
TEB Facility [Member]
Debt Instrument [Line Items]
Debt financing	93,988,000		94,933,000
Categories of Investments, Marketable Securities, Trading Securities [Member]
Debt Instrument [Line Items]
Debt financing	177,948,000		112,673,000
Debt Instrument, Face Amount	179,920,000		113,620,000
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB facility-2 [Member]
Debt Instrument [Line Items]
Debt financing	48,995,000
Debt Instrument, Face Amount	48,995,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	2.13%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB Facility-1 [Member]
Debt Instrument [Line Items]
Debt financing	9,850,000		9,930,000
Debt Instrument, Face Amount	10,000,000		10,000,000
Date of Debt Borrowing	2011		2011
Available-for-sale Security, Maturity Date at a point in time	Jul. 01, 2013		Jul. 01, 2012
Debt Instrument, Interest Rate, Stated Percentage	2.30%	[1]	2.01%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | TOB Facility-4 [Member]
Debt Instrument [Line Items]
Debt financing			7,810,000
Debt Instrument, Face Amount			7,810,000
Date of Debt Borrowing			2011
Available-for-sale Security, Maturity Date at a point in time			Nov. 01, 2012
Debt Instrument, Interest Rate, Stated Percentage			1.70%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | TEB Facility [Member]
Debt Instrument [Line Items]
Debt financing	93,988,000		94,933,000
Debt Instrument, Face Amount	95,810,000		95,810,000
Date of Debt Borrowing	2010		2010
Available-for-sale Security, Maturity Date at a point in time	Sep. 01, 2013		Sep. 01, 2017
Debt Instrument, Interest Rate, Stated Percentage	2.09%	[1]	2.10%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | Mortgage Backed Security Trust 1 [Member] | TOB facility-3 [Member]
Debt Instrument [Line Items]
Debt financing	2,585,000
Debt Instrument, Face Amount	2,585,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	1.31%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | Mortgage Backed Security Trust 2 [Member] | TOB facility-3 [Member]
Debt Instrument [Line Items]
Debt financing	4,090,000
Debt Instrument, Face Amount	4,090,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	1.29%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | Mortgage Backed Security Trust 3 [Member] | TOB facility-3 [Member]
Debt Instrument [Line Items]
Debt financing	3,890,000
Debt Instrument, Face Amount	3,890,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	1.32%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | Mortgage Backed Security Trust 4 [Member] | TOB facility-3 [Member]
Debt Instrument [Line Items]
Debt financing	5,960,000
Debt Instrument, Face Amount	5,960,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	1.29%	[1]
Categories of Investments, Marketable Securities, Trading Securities [Member] | Mortgage Backed Security Trust 5 [Member] [Member] | TOB facility-3 [Member]
Debt Instrument [Line Items]
Debt financing	8,590,000
Debt Instrument, Face Amount	$ 8,590,000
Date of Debt Borrowing	2012
Available-for-sale Security, Maturity Date at a point in time	Oct. 01, 2013
Debt Instrument, Interest Rate, Stated Percentage	1.28%	[1]

[1] Represents the average effective interest rate, including fees, for the years ended December 31, 2012 and 2011 and excludes the effect of interest rate caps (see Note 15).