Borrowings under Repurchase Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Reverse Repurchase Agreements [Line Items]
Non Mark-to-Market Reverse Repurchase Agreement Facility	150,000,000
Initial Period of Non-Mark-to-Market Financing Facility	18 months
Cancellation Notice Period Under Non-Mark-to-Market Financing Facility	6 months
Reverse repurchase agreements	1,669,433,000	[1]	1,236,166,000	[1]
Number of Counterparties with Outstanding Reverse Repurchase Agreements	16		14
Concentration Risk, Reverse Repurchase Agreements	23.00%		39.00%
Number Counterparties With Concentration Risk, Reverse Repurchase Agreements	1		2
Forward Starting Reverse Repurchase Agreements	50,100,000		48,700,000
Investments held with dealers as collateral	1,930,000,000		1,460,000,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	145,500,000		13,300,000
Minimum [Member]
Reverse Repurchase Agreements [Line Items]
Reverse Repurchase Agreements Maturity	30 days
Financing term under non-mark-to-market facility	2 years
Reverse Repurchase Agreements Remaining Terms	2 days		2 days
Reverse Repurchase Agreements Interest Rate	(1.50%)		0.32%
Maximum [Member]
Reverse Repurchase Agreements [Line Items]
Reverse Repurchase Agreements Maturity	180 days
Reverse Repurchase Agreements Remaining Terms	631 days		180 days
Reverse Repurchase Agreements Interest Rate	2.42%		2.27%
Reverse Repurchase Agreements [Member]
Reverse Repurchase Agreements [Line Items]
Cash Collateral (Received) Pledged	14,245,000	[2],[3]	2,926,000	[2],[3]
Security Owned and Pledged as Collateral, Fair Value	12,500,000		20,600,000

[1] In the Company's Consolidated Statement of Assets, Liabilities, and Equity, all balances associated with repurchase agreements, reverse repurchase agreements, and financial derivatives are presented on a gross basis.

[2] For the purpose of this presentation, for each row the total amount of financial instruments transferred or pledged and cash collateral (received) or pledged may not exceed the applicable gross amount of assets or (liabilities) as presented here. Therefore, the Company has reduced the amount of financial instruments transferred or pledged as collateral related to the Company's reverse repurchase agreements and cash collateral pledged on the Company's financial derivative liabilities. Total financial instruments transferred or pledged as collateral on the Company's reverse repurchase agreements as of December 31, 2014 and December 31, 2013 were $1.94 billion and $1.48 billion, respectively. As of December 31, 2014 and December 31, 2013, total cash collateral on financial derivative assets excludes excess net cash collateral pledged of $22.7 million and $3.2 million, respectively. As of December 31, 2014 and December 31, 2013, total cash collateral on financial derivative liabilities excludes excess cash collateral pledged of $23.2 million and $36.4 million, respectively.

[3] When collateral is pledged to or pledged by a counterparty, it is often pledged or posted with respect to all positions with such counterparty, and in such cases such collateral cannot be specifically identified as relating to a specific asset or liability. As a result, in preparing the above tables, the Company has made assumptions in allocating pledged or posted collateral among the various rows.