Schedule of Reverse Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Borrowings [Line Items]
Reverse repurchase agreements	[1]	$ 1,029,810	$ 1,033,581
Debt, Weighted Average Interest Rate		1.65%	1.32%
Remaining Days to Maturity, Reverse Repurchase Agreements		48 days	56 days
Agency residential mortgage-backed securities [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 768,418	$ 790,312
Debt, Weighted Average Interest Rate		1.33%	0.89%
Remaining Days to Maturity, Reverse Repurchase Agreements		43 days	41 days
Agency residential mortgage-backed securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 281,543	$ 405,725
Debt, Weighted Average Interest Rate		1.28%	0.83%
Remaining Days to Maturity, Reverse Repurchase Agreements		15 days	18 days
Agency residential mortgage-backed securities [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 263,601	$ 195,288
Debt, Weighted Average Interest Rate		1.33%	0.94%
Remaining Days to Maturity, Reverse Repurchase Agreements		44 days	45 days
Agency residential mortgage-backed securities [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 184,448	$ 149,965
Debt, Weighted Average Interest Rate		1.39%	0.97%
Remaining Days to Maturity, Reverse Repurchase Agreements		75 days	74 days
Agency residential mortgage-backed securities [Member] | Maturity 91 to 120 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 38,826	$ 8,240
Debt, Weighted Average Interest Rate		1.40%	0.83%
Remaining Days to Maturity, Reverse Repurchase Agreements		103 days	102 days
Agency residential mortgage-backed securities [Member] | Maturity 121 to 150 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 0	$ 11,798
Debt, Weighted Average Interest Rate		0.00%	0.96%
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	131 days
Agency residential mortgage-backed securities [Member] | Maturity 151 to 180 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 0	$ 19,296
Debt, Weighted Average Interest Rate		0.00%	1.05%
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	164 days
Credit Portfolio [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 255,672	$ 237,841
Debt, Weighted Average Interest Rate		2.61%	2.75%
Remaining Days to Maturity, Reverse Repurchase Agreements		60 days	105 days
Credit Portfolio [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 64,719	$ 94,849
Debt, Weighted Average Interest Rate		1.79%	2.55%
Remaining Days to Maturity, Reverse Repurchase Agreements		7 days	16 days
Credit Portfolio [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 83,826	$ 26,974
Debt, Weighted Average Interest Rate		2.77%	2.36%
Remaining Days to Maturity, Reverse Repurchase Agreements		47 days	47 days
Credit Portfolio [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 52,247	$ 41,522
Debt, Weighted Average Interest Rate		2.19%	2.43%
Remaining Days to Maturity, Reverse Repurchase Agreements		72 days	77 days
Credit Portfolio [Member] | Maturity 91 to 120 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 34,783	$ 10,084
Debt, Weighted Average Interest Rate		3.82%	2.91%
Remaining Days to Maturity, Reverse Repurchase Agreements		103 days	97 days
Credit Portfolio [Member] | Maturity 121 to 150 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 3,338	$ 1,239
Debt, Weighted Average Interest Rate		3.04%	2.73%
Remaining Days to Maturity, Reverse Repurchase Agreements		126 days	124 days
Credit Portfolio [Member] | Maturity 151 to 180 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 8,991	$ 12,616
Debt, Weighted Average Interest Rate		3.63%	3.17%
Remaining Days to Maturity, Reverse Repurchase Agreements		164 days	165 days
Credit Portfolio [Member] | Maturity 181 to 360 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 7,768	$ 50,557
Debt, Weighted Average Interest Rate		3.72%	3.46%
Remaining Days to Maturity, Reverse Repurchase Agreements		234 days	316 days
US Treasury Securities [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 5,720	$ 5,428
Debt, Weighted Average Interest Rate		1.30%	0.91%
Remaining Days to Maturity, Reverse Repurchase Agreements		2 days	4 days
US Treasury Securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 5,720	$ 5,428
Debt, Weighted Average Interest Rate		1.30%	0.91%
Remaining Days to Maturity, Reverse Repurchase Agreements		2 days	4 days

[1] In the Company's Consolidated Statement of Assets, Liabilities, and Equity, all balances associated with repurchase agreements, reverse repurchase agreements, and financial derivatives are presented on a gross basis.