Loans and Allowance for Losses and Concentrations of Credit Risk - Troubled Debt Restructurings (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 1,100,000	$ 4,500,000	$ 0	$ 5,300,000
Financing Receivable, Modifications, Post-Modification Recorded Investment	0	5,100,000	$ 1,100,000	6,000,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts			0
Financing Receivable, Modifications, Subsequent Default, Recorded Investment			$ 0
Total (release of)/provision for losses	(303,000)	(804,000)	257,000	(2,687,000)
Loans Collateralized by Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total (release of)/provision for losses	$ 0	$ 0	$ 0	$ 0