Derivatives and Hedging Activities - Credit Risk Related Contingent Features (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative liability, net	$ 20,354	$ 32,765
Cash margin deposits	5,800	620
Agency RMBS
Derivative [Line Items]
Cash margin deposits	8,400
Bilateral Counterparty
Derivative [Line Items]
Derivative liability, net	6,600
Central Clearing Counterparty
Derivative [Line Items]
Derivative liability, net	$ 12,200	$ 10,300