Schedule of Reverse Repurchase Agreements by Maturity (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements	[1]	$ 1,372,794	$ 1,669,433
Debt, Weighted Average Interest Rate		0.78%	0.65%
Remaining Days to Maturity, Reverse Repurchase Agreements		76 days	105 days
Agency residential mortgage-backed securities [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 1,145,839	$ 1,145,821
Debt, Weighted Average Interest Rate		0.48%	0.35%
Remaining Days to Maturity, Reverse Repurchase Agreements		52 days	50 days
Agency residential mortgage-backed securities [Member] | Maturity up to 30 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 389,834	$ 537,624
Debt, Weighted Average Interest Rate		0.45%	0.34%
Remaining Days to Maturity, Reverse Repurchase Agreements		14 days	16 days
Agency residential mortgage-backed securities [Member] | Maturity 31 to 60 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 272,247	$ 276,002
Debt, Weighted Average Interest Rate		0.48%	0.34%
Remaining Days to Maturity, Reverse Repurchase Agreements		45 days	44 days
Agency residential mortgage-backed securities [Member] | Maturity 61 to 90 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 290,038	$ 251,715
Debt, Weighted Average Interest Rate		0.49%	0.38%
Remaining Days to Maturity, Reverse Repurchase Agreements		72 days	72 days
Agency residential mortgage-backed securities [Member] | Maturity 91 to 120 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 191,974	$ 0
Debt, Weighted Average Interest Rate		0.53%	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]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 1,746	$ 48,161
Debt, Weighted Average Interest Rate		1.06%	0.44%
Remaining Days to Maturity, Reverse Repurchase Agreements		173 days	164 days
Agency residential mortgage-backed securities [Member] | Maturity 181 to 360 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 0	$ 32,319
Debt, Weighted Average Interest Rate		0.00%	0.47%
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	317 days
Credit Portfolio [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 226,955	$ 399,981
Debt, Weighted Average Interest Rate		2.28%	2.02%
Remaining Days to Maturity, Reverse Repurchase Agreements		194 days	295 days
Credit Portfolio [Member] | Maturity up to 30 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 23,119	$ 53,939
Debt, Weighted Average Interest Rate		1.57%	1.94%
Remaining Days to Maturity, Reverse Repurchase Agreements		17 days	22 days
Credit Portfolio [Member] | Maturity 31 to 60 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 25,571	$ 46,872
Debt, Weighted Average Interest Rate		1.84%	1.59%
Remaining Days to Maturity, Reverse Repurchase Agreements		41 days	43 days
Credit Portfolio [Member] | Maturity 61 to 90 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 21,250	$ 37,561
Debt, Weighted Average Interest Rate		1.94%	1.47%
Remaining Days to Maturity, Reverse Repurchase Agreements		68 days	65 days
Credit Portfolio [Member] | Maturity 91 to 120 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 33,860	$ 0
Debt, Weighted Average Interest Rate		2.69%	0.00%
Remaining Days to Maturity, Reverse Repurchase Agreements		100 days	0 days
Credit Portfolio [Member] | Maturity 121 to 150 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 4,106	$ 21,236
Debt, Weighted Average Interest Rate		1.70%	2.03%
Remaining Days to Maturity, Reverse Repurchase Agreements		145 days	139 days
Credit Portfolio [Member] | Maturity 151 to 180 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 20,723	$ 75,323
Debt, Weighted Average Interest Rate		2.10%	1.77%
Remaining Days to Maturity, Reverse Repurchase Agreements		163 days	160 days
Credit Portfolio [Member] | Maturity 181 to 360 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 98,326	$ 15,449
Debt, Weighted Average Interest Rate		2.47%	1.65%
Remaining Days to Maturity, Reverse Repurchase Agreements		344 days	181 days
Credit Portfolio [Member] | Maturity greater than 360 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 0	$ 149,601
Debt, Weighted Average Interest Rate		0.00%	2.41%
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	631 days
US Treasury Securities [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 0	$ 123,631
Debt, Weighted Average Interest Rate		0.00%	(1.01%)
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	2 days
US Treasury Securities [Member] | Maturity up to 30 days [Member]
Reverse Repurchase Agreements [Line Items]
Reverse repurchase agreements		$ 0	$ 123,631
Debt, Weighted Average Interest Rate		0.00%	(1.01%)
Remaining Days to Maturity, Reverse Repurchase Agreements		0 days	2 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.