Derivative Instruments - Notional Value, Fair Value and Gains and Losses Recognized in Investment Gains (Losses) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Value	$ 664,558	$ 508,405
Derivative Assets	12,588	6,787
Derivative Liabilities	12,221	9,392
Gains (Losses)	15,172	(19,277)
Exchange-Traded Future [Member]
Derivative [Line Items]
Notional Value	160,755	149,863
Derivative Assets	1,539	571
Derivative Liabilities	2,651	2,438
Gains (Losses)	8,572	(3,766)
Currency Forwards [Member]
Derivative [Line Items]
Notional Value	262,873	149,282
Derivative Assets	4,604	1,782
Derivative Liabilities	4,077	333
Gains (Losses)	7,445	3,160
Interest Rate Swap [Member]
Derivative [Line Items]
Notional Value	65,484	50,591
Derivative Assets	2,945	1,507
Derivative Liabilities	3,745	2,679
Gains (Losses)	(443)	(2,941)
Credit Default Swap [Member]
Derivative [Line Items]
Notional Value	29,421	32,745
Derivative Assets	2,089	1,432
Derivative Liabilities	774	110
Gains (Losses)	(253)	(826)
Option swap [Member]
Derivative [Line Items]
Notional Value	24	11
Derivative Assets	9	107
Derivative Liabilities	2	88
Gains (Losses)	11	(338)
Total Return Swap [Member]
Derivative [Line Items]
Notional Value	146,001	125,913
Derivative Assets	1,402	1,388
Derivative Liabilities	972	3,744
Gains (Losses)	$ (160)	$ (14,566)