Carrying Values and Fair Values of All Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013	Dec. 31, 2012
Fair Value Of Financial Instruments [Line Items]
Mortgage-backed securities	$ 17,137,956		$ 17,642,532
Cash and cash equivalents	250,258		763,326		315,253	168,424
Restricted cash	231,591		225,379
Unsettled purchased mortgage-backed securities, at fair value	157,201
Accrued interest receivable	53,629		55,156
Receivable for securities sold			231,214
Principal payments receivable	70,524		95,021
Debt security, held to maturity	15,000		15,000
Interest rate swap asset	12,703		15,841
Futures Contracts	5,617		11,148
Forward purchase commitments	2,042		0
Repurchase agreements	15,183,457		16,129,683
Dollar roll liabilities			351,826
Payable for unsettled securities	157,387
Accrued interest payable	3,475		8,279
Interest rate swap liability	106,132		125,133
Futures contract liability	48,584		36,733
Forward purchase commitments	13,634		5,741
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value Of Financial Instruments [Line Items]
Mortgage-backed securities	17,137,956		17,642,532
Cash and cash equivalents	250,258		763,326
Restricted cash	231,591		225,379
Unsettled purchased mortgage-backed securities, at fair value	157,201		0
Accrued interest receivable	53,629		55,156
Receivable for securities sold	0		231,214
Principal payments receivable	70,524		95,021
Debt security, held to maturity	15,000		15,000
Interest rate swap asset	12,703		15,841
Futures Contracts	5,617	[1]	11,148	[1]
Short term investment	20,052	[1]	19,910	[1]
Forward purchase commitments	2,042	[1]	0	[1]
Repurchase agreements	15,183,457		16,129,683
Dollar roll liabilities	0		351,826
Payable for unsettled securities	157,387		0
Accrued interest payable	3,475		8,279
Interest rate swap liability	106,132		125,133
Futures contract liability	48,584		36,733
Forward purchase commitments	13,634	[2]	5,741	[2]
Estimate of Fair Value, Fair Value Disclosure
Fair Value Of Financial Instruments [Line Items]
Mortgage-backed securities	17,137,956		17,642,532
Cash and cash equivalents	250,258		763,326
Restricted cash	231,591		225,379
Unsettled purchased mortgage-backed securities, at fair value	157,201		0
Accrued interest receivable	53,629		55,156
Receivable for securities sold	0		231,214
Principal payments receivable	70,524		95,021
Debt security, held to maturity	14,482		14,307
Interest rate swap asset	12,703		15,841
Futures Contracts	5,617	[1]	11,148	[1]
Short term investment	20,052	[1]	19,910	[1]
Forward purchase commitments	2,042	[1]	0	[1]
Repurchase agreements	15,183,457		16,129,683
Dollar roll liabilities	0		351,826
Payable for unsettled securities	157,387		0
Accrued interest payable	3,475		8,279
Interest rate swap liability	106,132		125,133
Futures contract liability	48,584		36,733
Forward purchase commitments	$ 13,634	[2]	$ 5,741	[2]

[1] These lines are included in other assets on the consolidated balance sheets.

[2] This line is included in accounts payable and other liabilities on the consolidated balance sheets.