Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Assets:
Agency MBS	$ 6,519,221,000	$ 7,225,460,000
Securities obtained and pledged as collateral	383,743,000	408,833,000
Linked Transactions	28,181,000	12,704,000
Derivative hedging instruments	13,000,000	203,000
Liabilities:
Obligation to return securities obtained as collateral	383,743,000	508,827,000
Level 2
Liabilities:
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	0
Recurring basis | Level 1
Assets:
Agency MBS	0
Non-Agency MBS, including MBS transferred to consolidated VIEs	0
Securities obtained and pledged as collateral	383,743,000
Linked Transactions	0
Derivative hedging instruments	0
Total assets carried at fair value	383,743,000
Liabilities:
Derivative hedging instruments	0
Obligation to return securities obtained as collateral	383,743,000
Total liabilities carried at fair value	383,743,000
Recurring basis | Level 2
Assets:
Agency MBS	6,519,221,000
Non-Agency MBS, including MBS transferred to consolidated VIEs	4,852,137,000
Securities obtained and pledged as collateral	0
Linked Transactions	28,181,000
Derivative hedging instruments	13,000,000
Total assets carried at fair value	11,412,539,000
Liabilities:
Derivative hedging instruments	28,217,000
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	28,217,000
Recurring basis | Level 3
Assets:
Agency MBS	0
Non-Agency MBS, including MBS transferred to consolidated VIEs	0
Securities obtained and pledged as collateral	0
Linked Transactions	0
Derivative hedging instruments	0
Total assets carried at fair value	0
Liabilities:
Derivative hedging instruments	0
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	0
Recurring basis | Estimated fair value
Assets:
Agency MBS	6,519,221,000
Non-Agency MBS, including MBS transferred to consolidated VIEs	4,852,137,000
Securities obtained and pledged as collateral	383,743,000
Linked Transactions	28,181,000
Derivative hedging instruments	13,000,000
Total assets carried at fair value	11,796,282,000
Liabilities:
Derivative hedging instruments	28,217,000
Obligation to return securities obtained as collateral	383,743,000
Total liabilities carried at fair value	$ 411,960,000