Loan Allowance Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	$ 493	$ 580	$ 669
Provision for credit losses under letters of credit	6,351	8,080	9,899
Amounts paid to Independent Lender under letters of credit, net of recoveries from customers	(6,346)	(8,167)	(9,988)
Balance at end of year	$ 498	$ 493	$ 580