Borrowings Under Repurchase Agreements - Maturity (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Borrowings under repurchase agreements
Repurchase Agreement Borrowings	$ 520,169	$ 600,852
Weighted Average Interest Rate	3.80%	1.80%
Within 30 days
Borrowings under repurchase agreements
Repurchase Agreement Borrowings	$ 117,106	$ 350,054
Weighted Average Interest Rate	1.50%	0.40%
Over 30 days to 60 days
Borrowings under repurchase agreements
Repurchase Agreement Borrowings	$ 15,445	$ 87,942
Weighted Average Interest Rate	3.10%	4.00%
Over 60 days to 90 days
Borrowings under repurchase agreements
Repurchase Agreement Borrowings	$ 18,557	$ 8,112
Weighted Average Interest Rate	3.10%	2.90%
Over 120 days to 360 days
Borrowings under repurchase agreements
Repurchase Agreement Borrowings	$ 369,061	$ 154,744
Weighted Average Interest Rate	4.60%	3.60%