Fair Value of Financial Instruments (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets:
Agency MBS	$ 6,697,689,000	$ 7,225,460,000
Securities obtained and pledged as collateral	252,796,000	408,833,000
Linked Transactions	23,940,000	12,704,000
Other derivative instruments	4,629,000	203,000
Liabilities:
Obligation to return securities obtained as collateral	252,796,000	508,827,000
Level 2
Components of financial instruments carried at fair value
Credit valuation adjustment to derivative liabilities	0
Credit valuation adjustment to derivative assets	0
Recurring basis
Assets:
Agency MBS	6,697,689,000
Non-Agency MBS, including MBS transferred to consolidated VIEs	4,973,378,000
Securities obtained and pledged as collateral	252,796,000
Linked Transactions	23,940,000
Other derivative instruments	4,629,000
Total assets carried at fair value	11,952,432,000
Liabilities:
Other derivative instruments	65,253,000
Obligation to return securities obtained as collateral	252,796,000
Total liabilities carried at fair value	318,049,000
Recurring basis | Level 1
Assets:
Agency MBS	0
Non-Agency MBS, including MBS transferred to consolidated VIEs	0
Securities obtained and pledged as collateral	252,796,000
Linked Transactions	0
Other derivative instruments	0
Total assets carried at fair value	252,796,000
Liabilities:
Other derivative instruments	0
Obligation to return securities obtained as collateral	252,796,000
Total liabilities carried at fair value	252,796,000
Recurring basis | Level 2
Assets:
Agency MBS	6,697,689,000
Non-Agency MBS, including MBS transferred to consolidated VIEs	4,973,378,000
Securities obtained and pledged as collateral	0
Linked Transactions	23,940,000
Other derivative instruments	4,629,000
Total assets carried at fair value	11,699,636,000
Liabilities:
Other derivative instruments	65,253,000
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	65,253,000
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
Other derivative instruments	0
Total assets carried at fair value	0
Liabilities:
Other derivative instruments	0
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	$ 0