Carrying Values and Fair Values of All Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
MBS	$ 17,587,010		$ 17,642,532
Mortgage loans held for investment, at fair value	31,460
Unsettled purchased MBS	24,792
Receivable for securities sold	5,197		231,214
Accrued interest receivable	54,274		55,156
Principal payments receivable	111,439		95,021
Debt security, held-to-maturity	15,000		15,000
Short-term investments	20,252		19,910
Repurchase agreements	15,759,831		16,129,683
Dollar roll liability			351,826
Payable for unsettled securities	24,750
Accrued interest payable	6,968		8,279
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
MBS	17,587,010		17,642,532
Mortgage loans held for investment, at fair value	31,460
Cash and cash equivalents	627,595		988,705
Unsettled purchased MBS	24,792
Receivable for securities sold	5,197		231,214
Accrued interest receivable	54,274		55,156
Principal payments receivable	111,439		95,021
Debt security, held-to-maturity	15,000	[1]	15,000	[1]
Short-term investments	20,252	[1]	19,910	[1]
Interest rate swaps and swaptions	11,050	[2]	15,841	[2]
Futures Contracts asset			11,148	[2]
Forward purchase commitments	16,101	[2]
Repurchase agreements	15,759,831		16,129,683
Dollar roll liability			351,826
Payable for unsettled securities	24,750
Accrued interest payable	6,968		8,279
Interest rate swap liability	42,052	[3]	125,133	[3]
Futures Contracts liability	202,501	[3]	36,733	[3]
Forward purchase commitments	38	[3]	5,741	[3]
Estimate of Fair Value, Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
MBS	17,587,010		17,642,532
Mortgage loans held for investment, at fair value	31,460
Cash and cash equivalents	627,595		988,705
Unsettled purchased MBS	24,792
Receivable for securities sold	5,197		231,214
Accrued interest receivable	54,274		55,156
Principal payments receivable	111,439		95,021
Debt security, held-to-maturity	14,853	[1]	14,307	[1]
Short-term investments	20,252	[1]	19,910	[1]
Interest rate swaps and swaptions	11,050	[2]	15,841	[2]
Futures Contracts asset			11,148	[2]
Forward purchase commitments	16,101	[2]
Repurchase agreements	15,759,831		16,129,683
Dollar roll liability			351,826
Payable for unsettled securities	24,750
Accrued interest payable	6,968		8,279
Interest rate swap liability	42,052	[3]	125,133	[3]
Futures Contracts liability	202,501	[3]	36,733	[3]
Forward purchase commitments	$ 38	[3]	$ 5,741	[3]

[1] Included in other investments on the consolidated balance sheets.

[2] Included in derivative assets on the consolidated balance sheets.

[3] Included in derivative liabilities on the consolidated balance sheets.