Repurchase Agreements Counterparty Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Agreements
Repurchase Agreement Counterparty [Line Items]
Borrowings outstanding with counterparty	$ 311,351
Weighted Average Rate	2.43%
Equity at Risk	$ 56,383
Line of Credit Facility, Amount Outstanding	304,005
Line of Credit Facility, Maximum Borrowing Capacity	$ 400,000
Line of Credit Facility, Expiration Date	May 12, 2019
Line of Credit Facility, Interest Rate at Period End	2.43%
Number of Counterparties with Borrowings Outstanding | Agreements	16
Available Repurchase Agreement Counterparties | Agreements	34