Borrowings - Other Secured Debt (Details) (USD $)	0 Months Ended
	Dec. 28, 2014	Sep. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Feb. 09, 2015	Nov. 25, 2014	Sep. 25, 2014
Debt Instrument [Line Items]
Other secured borrowings			$ 379,840,000	$ 339,082,000
Securities sold under agreements to repurchase		15,000,000	14,991,000	14,991,000
Purchase price of securities sold under agreements to repurchase		15,000,000
Number of days master repurchase agreement may be extended	89 days
Asset-backed securities Class A notes Trust 2
Debt Instrument [Line Items]
Other secured borrowings			264,034,000	269,820,000	50,700,000	270,800,000
Secured debt issued to affiliates			45,138,000	95,729,000		95,800,000
Interest rate on debt			3.85%	3.85%		3.85%
Asset-backed securities Class M notes Trust 2
Debt Instrument [Line Items]
Other secured borrowings				234,010,000		234,000,000
Secured debt issued to affiliates			(234,010,000)	234,010,000
Interest rate on debt			0.00%	0.00%
Asset-backed securities Class A notes
Debt Instrument [Line Items]
Other secured borrowings			145,953,000	150,000,000			150,000,000
Interest rate on debt			3.47%	3.47%			3.47%
Asset-backed securities Class M notes
Debt Instrument [Line Items]
Other secured borrowings			32,000,000	32,000,000			32,000,000
Secured debt issued to affiliates			(32,000,000)	32,000,000
Interest rate on debt			4.25%	4.25%			4.25%
Repurchase agreement NewSource
Debt Instrument [Line Items]
Securities sold under agreement to repurchase, interest rate	5.00%		5.00%	5.00%
ARLP 2014-2 | Loans
Debt Instrument [Line Items]
Book value of the underlying securitized assets			332,500,000
ARLP 2014-1 | Loans
Debt Instrument [Line Items]
Book value of the underlying securitized assets			$ 213,800,000