Loans and Allowance for Losses and Concentrations of Credit Risk - Troubled Debt Restructurings (Details) defaulted_loan in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) contract	Jun. 30, 2014 USD ($) defaulted_loan	Dec. 31, 2014 USD ($)
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment		$ 1,100,000	$ 300,000	$ 1,100,000
Financing Receivable, Modifications, Post-Modification Recorded Investment		1,100,000	800,000		$ 800,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts				0	0.3
Financing Receivable, Modifications, Subsequent Default, Recorded Investment				$ 1,100,000
Total (release of)/provision for losses		1,256,000	(2,557,000)	560,000	$ (1,883,000)
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities, Subject To Be Purchased	[1]	1,800,000		1,800,000		$ 1,800,000
Loans Collateralized by Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Total (release of)/provision for losses		$ 0	$ 0	$ 0	$ 0

[1] Includes loans and loans underlying off-balance sheet Farm & Ranch Guaranteed Securities and LTSPCs that are 90 days or more past due, in foreclosure, or in bankruptcy, excluding loans performing under either their original loan terms or a court-approved bankruptcy plan.