Note 10 - Fair Value Accounting - Significant Unobservable Inputs (Details)	Dec. 31, 2017
Servicer's optional redemption date	0.00%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Measurement Input, Loss Severity [Member]
Measurement input	0.621
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Measurement Input, Default Rate [Member]
Measurement input	0.02
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Measurement Input, Prepayment Rate [Member]
Measurement input	0.135