Fair Value of Financial Instruments (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013
Assets:
Available-for-sale Securities	$ 10,937,502,000		$ 11,371,358,000
Securities obtained and pledged as collateral	442,370,000		383,743,000
Linked Transactions	190,681,000		28,181,000
Swaps	4,322,000		13,000,000
Liabilities:
Swaps	35,493,000	[1]	28,217,000	[1]
Obligation to return securities obtained as collateral	442,370,000		383,743,000
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 | Level 1
Assets:
Securities obtained and pledged as collateral	442,370,000
Linked Transactions	0
Swaps	0
Total assets carried at fair value	442,370,000
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	442,370,000
Total liabilities carried at fair value	442,370,000
Recurring basis | Level 2
Assets:
Securities obtained and pledged as collateral	0
Linked Transactions	190,681,000
Swaps	4,322,000
Total assets carried at fair value	11,132,505,000
Liabilities:
Swaps	35,493,000
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	35,493,000
Recurring basis | Level 3
Assets:
Securities obtained and pledged as collateral	0
Linked Transactions	0
Swaps	0
Total assets carried at fair value	0
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	0
Recurring basis | Estimated Fair Value
Assets:
Securities obtained and pledged as collateral	442,370,000
Linked Transactions	190,681,000
Swaps	4,322,000
Total assets carried at fair value	11,574,875,000
Liabilities:
Swaps	35,493,000
Obligation to return securities obtained as collateral	442,370,000
Total liabilities carried at fair value	477,863,000
Agency Mortgage Backed Securities
Assets:
Available-for-sale Securities	6,174,176,000		6,519,221,000
Agency Mortgage Backed Securities | Recurring basis | Level 1
Assets:
Available-for-sale Securities	0
Agency Mortgage Backed Securities | Recurring basis | Level 2
Assets:
Available-for-sale Securities	6,174,176,000
Agency Mortgage Backed Securities | Recurring basis | Level 3
Assets:
Available-for-sale Securities	0
Agency Mortgage Backed Securities | Recurring basis | Estimated Fair Value
Assets:
Available-for-sale Securities	6,174,176,000
Non-Agency Mortgage Backed Securities
Assets:
Available-for-sale Securities	4,763,326,000		4,852,137,000
Non-Agency Mortgage Backed Securities | Recurring basis | Level 1
Assets:
Available-for-sale Securities	0
Non-Agency Mortgage Backed Securities | Recurring basis | Level 2
Assets:
Available-for-sale Securities	4,763,326,000
Non-Agency Mortgage Backed Securities | Recurring basis | Level 3
Assets:
Available-for-sale Securities	0
Non-Agency Mortgage Backed Securities | Recurring basis | Estimated Fair Value
Assets:
Available-for-sale Securities	$ 4,763,326,000

[1] The fair value of securities pledged against the Company’s Swaps was $67.4 million and $73.9 million at September 30, 2014 and December 31, 2013, respectively.