Borrowings under Repurchase Agreements (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
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
Number of Counterparties with Outstanding Reverse Repurchase Agreements	16		16
Concentration Risk, Reverse Repurchase Agreements	37.00%		23.00%
Number Counterparties With Concentration Risk, Reverse Repurchase Agreements	2		1
Forward Starting Reverse Repurchase Agreements	13,900,000		50,100,000
Investments held with dealers as collateral	1,620,000,000		1,930,000,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled	35,900,000		145,500,000
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	1 day		2 days
Reverse Repurchase Agreements Interest Rate	(0.22%)		(1.50%)
Maximum [Member]
Reverse Repurchase Agreements [Line Items]
Reverse Repurchase Agreements Maturity	180 days
Reverse Repurchase Agreements Remaining Terms	541 days		631 days
Reverse Repurchase Agreements Interest Rate	2.43%		2.42%
Reverse Repurchase Agreements [Member]
Reverse Repurchase Agreements [Line Items]
Cash Collateral (Received) Pledged	14,026,000	[1],[2]	14,245,000	[1],[2]
Security Owned and Pledged as Collateral, Fair Value	2,500,000		12,500,000

[1] 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 March 31, 2015 and December 31, 2014 were $1.63 billion and $1.94 billion, respectively. As of March 31, 2015 and December 31, 2014, total cash collateral on financial derivative assets excludes excess net cash collateral pledged of $17.6 million and $22.7 million, respectively. As of March 31, 2015 and December 31, 2014, total cash collateral on financial derivative liabilities excludes excess cash collateral pledged of $40.3 million and $23.2 million, respectively.

[2] 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.