Allowance for Credit Losses - Summary of Allowance for Credit Losses (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Allowance for credit losses and recorded investment in financing receivables
Financing Receivable, Allowance for Credit Losses, Provisions	$ 8	$ 261
Financing Receivable, Allowance for Credit Losses, Recoveries	3	48
Financing Receivable, Allowance for Credit Losses, Charge-offs	(2,137)	0
Financing Receivable, Allowance for Credit Losses	446	2,566	2,572	2,257
Finance Leases Financing Receivable [Member]
Allowance for credit losses and recorded investment in financing receivables
Financing Receivable, Allowance for Credit Losses, Provisions	8	261
Financing Receivable, Allowance for Credit Losses, Recoveries	3	48
Financing Receivable, Allowance for Credit Losses, Charge-offs	(90)	0
Financing Receivable, Allowance for Credit Losses	446	519	525	210
Balance individually evaluated for impairment	88,216	100,436
Notes Receivable [Member]
Allowance for credit losses and recorded investment in financing receivables
Financing Receivable, Allowance for Credit Losses, Provisions	0	0
Financing Receivable, Allowance for Credit Losses, Recoveries	0	0
Financing Receivable, Allowance for Credit Losses, Charge-offs	(2,047)	0
Financing Receivable, Allowance for Credit Losses	0	2,047	2,047	2,047
Balance individually evaluated for impairment	$ 16,343	$ 12,882