Schedule of Reverse Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Borrowings [Line Items]
Reverse repurchase agreements	[1]	$ 1,636,039	$ 1,209,315
Debt, Weighted Average Interest Rate		2.88%	1.98%
Remaining Days to Maturity, Reverse Repurchase Agreements		96 days	99 days
Agency residential mortgage-backed securities [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 901,685	$ 829,624
Debt, Weighted Average Interest Rate		2.29%	1.51%
Remaining Days to Maturity, Reverse Repurchase Agreements		49 days	44 days
Agency residential mortgage-backed securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 209,417	$ 287,014
Debt, Weighted Average Interest Rate		2.21%	1.43%
Remaining Days to Maturity, Reverse Repurchase Agreements		15 days	15 days
Agency residential mortgage-backed securities [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 428,517	$ 264,058
Debt, Weighted Average Interest Rate		2.28%	1.47%
Remaining Days to Maturity, Reverse Repurchase Agreements		46 days	46 days
Agency residential mortgage-backed securities [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 234,616	$ 277,950
Debt, Weighted Average Interest Rate		2.39%	1.63%
Remaining Days to Maturity, Reverse Repurchase Agreements		77 days	74 days
Agency residential mortgage-backed securities [Member] | Maturity 91 to 120 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 29,135	$ 0
Debt, Weighted Average Interest Rate		2.30%	0.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		107 days	0 days
Agency residential mortgage-backed securities [Member] | Maturity 151 to 180 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 0	$ 602
Debt, Weighted Average Interest Rate		0.00%	2.56%
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	158 days
US Treasury Securities [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 50,731	$ 297
Debt, Weighted Average Interest Rate		2.39%	1.70%
Remaining Days to Maturity, Reverse Repurchase Agreements		1 day	2 days
US Treasury Securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 50,731	$ 297
Debt, Weighted Average Interest Rate		2.39%	1.70%
Remaining Days to Maturity, Reverse Repurchase Agreements		1 day	2 days
Credit [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 683,623	$ 379,394
Debt, Weighted Average Interest Rate		3.68%	3.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		165 days	219 days
Credit [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 26,546	$ 37,433
Debt, Weighted Average Interest Rate		2.55%	2.61%
Remaining Days to Maturity, Reverse Repurchase Agreements		5 days	13 days
Credit [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 146,424	$ 132,201
Debt, Weighted Average Interest Rate		3.08%	2.44%
Remaining Days to Maturity, Reverse Repurchase Agreements		44 days	49 days
Credit [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 188,582	$ 130,875
Debt, Weighted Average Interest Rate		2.87%	2.75%
Remaining Days to Maturity, Reverse Repurchase Agreements		77 days	77 days
Credit [Member] | Maturity 91 to 120 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 125,812	$ 0
Debt, Weighted Average Interest Rate		4.14%	0.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		102 days	0 days
Credit [Member] | Maturity 121 to 150 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 2,593	$ 8,551
Debt, Weighted Average Interest Rate		4.14%	3.79%
Remaining Days to Maturity, Reverse Repurchase Agreements		142 days	128 days
Credit [Member] | Maturity 151 to 180 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 26,389	$ 8,300
Debt, Weighted Average Interest Rate		3.81%	3.40%
Remaining Days to Maturity, Reverse Repurchase Agreements		164 days	164 days
Credit [Member] | Maturity 181 to 360 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 103,881	$ 5,090
Debt, Weighted Average Interest Rate		4.51%	3.59%
Remaining Days to Maturity, Reverse Repurchase Agreements		214 days	280 days
Credit [Member] | Maturity greater than 360 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 63,396	$ 56,944
Debt, Weighted Average Interest Rate		5.62%	4.94%
Remaining Days to Maturity, Reverse Repurchase Agreements		821 days	1094 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.