Repurchase Agreements Repurchase Agreements, Counterparty Information (Details) $ in Thousands	Jun. 30, 2017 USD ($) Agreements
Counterparty Information [Line Items]
Line of Credit Facility, Amount Outstanding	$ 359,263
Line of Credit Facility, Maximum Borrowing Capacity	$ 400,000
Line of Credit Facility, Interest Rate at Period End	2.09%
Number of Counterparties with Borrowings Outstanding | Agreements	18
Available Repurchase Agreement Counterparties | Agreements	34
Wells Fargo Bank, N. A. and affiliates
Counterparty Information [Line Items]
Borrowings outstanding with counterparty	$ 371,660
WAVG interest rate for amount outstanding with named counterparty	2.09%
Repurchase Agreement Counterparty, Amount at Risk	$ 65,366