Schedule of Reverse Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Borrowings [Line Items]
Reverse repurchase agreements	[1]	$ 1,421,506	$ 1,209,315
Debt, Weighted Average Interest Rate		2.70%	1.98%
Remaining Days to Maturity, Reverse Repurchase Agreements		104 days	99 days
Agency residential mortgage-backed securities [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 891,082	$ 829,624
Debt, Weighted Average Interest Rate		2.12%	1.51%
Remaining Days to Maturity, Reverse Repurchase Agreements		45 days	44 days
Agency residential mortgage-backed securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 261,209	$ 287,014
Debt, Weighted Average Interest Rate		2.01%	1.43%
Remaining Days to Maturity, Reverse Repurchase Agreements		16 days	15 days
Agency residential mortgage-backed securities [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 395,408	$ 264,058
Debt, Weighted Average Interest Rate		2.13%	1.47%
Remaining Days to Maturity, Reverse Repurchase Agreements		45 days	46 days
Agency residential mortgage-backed securities [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 233,892	$ 277,950
Debt, Weighted Average Interest Rate		2.22%	1.63%
Remaining Days to Maturity, Reverse Repurchase Agreements		77 days	74 days
Agency residential mortgage-backed securities [Member] | Maturity 121 to 150 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 573	$ 0
Debt, Weighted Average Interest Rate		3.32%	0.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		125 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		$ 2,639	$ 297
Debt, Weighted Average Interest Rate		2.30%	1.70%
Remaining Days to Maturity, Reverse Repurchase Agreements		2 days	2 days
US Treasury Securities [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 2,639	$ 297
Debt, Weighted Average Interest Rate		2.30%	1.70%
Remaining Days to Maturity, Reverse Repurchase Agreements		2 days	2 days
Credit [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 527,785	$ 379,394
Debt, Weighted Average Interest Rate		3.69%	3.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		206 days	219 days
Credit [Member] | Maturity up to 30 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 25,011	$ 37,433
Debt, Weighted Average Interest Rate		2.62%	2.61%
Remaining Days to Maturity, Reverse Repurchase Agreements		3 days	13 days
Credit [Member] | Maturity 31 to 60 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 155,309	$ 132,201
Debt, Weighted Average Interest Rate		3.10%	2.44%
Remaining Days to Maturity, Reverse Repurchase Agreements		48 days	49 days
Credit [Member] | Maturity 61 to 90 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 144,983	$ 130,875
Debt, Weighted Average Interest Rate		3.33%	2.75%
Remaining Days to Maturity, Reverse Repurchase Agreements		76 days	77 days
Credit [Member] | Maturity 121 to 150 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 1,711	$ 8,551
Debt, Weighted Average Interest Rate		3.88%	3.79%
Remaining Days to Maturity, Reverse Repurchase Agreements		146 days	128 days
Credit [Member] | Maturity 151 to 180 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 12,241	$ 8,300
Debt, Weighted Average Interest Rate		4.17%	3.40%
Remaining Days to Maturity, Reverse Repurchase Agreements		167 days	164 days
Credit [Member] | Maturity 181 to 360 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 121,971	$ 5,090
Debt, Weighted Average Interest Rate		4.00%	3.59%
Remaining Days to Maturity, Reverse Repurchase Agreements		221 days	280 days
Credit [Member] | Maturity greater than 360 days [Member]
Borrowings [Line Items]
Reverse repurchase agreements		$ 66,559	$ 56,944
Debt, Weighted Average Interest Rate		5.59%	4.94%
Remaining Days to Maturity, Reverse Repurchase Agreements		913 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.