Borrowings under Repurchase Agreements (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) counterparties	Dec. 31, 2014 USD ($) counterparties
Reverse Repurchase Agreements [Line Items]
Number of Counterparties with Outstanding Reverse Repurchase Agreements | counterparties		18	16
Concentration Risk, Reverse Repurchase Agreements		15.00%	23.00%
Number Counterparties With Concentration Risk, Reverse Repurchase Agreements | counterparties		1	1
Forward Starting Reverse Repurchase Agreements		$ 4,400	$ 50,100
Investments held with dealers as collateral		1,350,000	1,930,000
Pledged Financial Instruments, Not Separately Reported, Securities For Repurchase Agreements Unsettled		$ 16,800	$ 145,500
Minimum [Member]
Reverse Repurchase Agreements [Line Items]
Reverse Repurchase Agreements Maturity		30 days
Reverse Repurchase Agreements Remaining Terms		4 days	2 days
Reverse Repurchase Agreements Interest Rate		0.37%	(1.50%)
Maximum [Member]
Reverse Repurchase Agreements [Line Items]
Reverse Repurchase Agreements Maturity		180 days
Reverse Repurchase Agreements Remaining Terms		666 days	631 days
Reverse Repurchase Agreements Interest Rate		2.92%	2.42%
Reverse Repurchase Agreements [Member]
Reverse Repurchase Agreements [Line Items]
Cash collateral posted for securities sold under agreements to repurchase	[1],[2]	$ 25,542	$ 14,245
Security Owned and Pledged as Collateral, Fair Value		$ 5,100	$ 12,500

[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 December 31, 2015 and 2014 were $1.36 billion and $1.94 billion, respectively. As of December 31, 2015 and 2014, total cash collateral on financial derivative assets excludes excess net cash collateral pledged of $4.1 million and $22.7 million, respectively. As of December 31, 2015 and 2014, total cash collateral on financial derivative liabilities excludes excess cash collateral pledged of $45.8 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.