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, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional Value	$ 459,743	$ 664,558
Derivative Assets	8,413	12,588
Derivative Liabilities	8,649	12,221
Gains (Losses)	(15,528)	15,172
Exchange-Traded Future [Member]
Derivative [Line Items]
Notional Value	103,108	160,755
Derivative Assets	1,224	1,539
Derivative Liabilities	1,092	2,651
Gains (Losses)	(2,754)	8,572
Currency Forwards [Member]
Derivative [Line Items]
Notional Value	180,820	262,873
Derivative Assets	4,541	4,604
Derivative Liabilities	4,711	4,077
Gains (Losses)	(2,028)	7,445
Interest Rate Swap [Member]
Derivative [Line Items]
Notional Value	40,664	65,484
Derivative Assets	940	2,945
Derivative Liabilities	897	3,745
Gains (Losses)	(572)	(443)
Credit Default Swap [Member]
Derivative [Line Items]
Notional Value	45,108	29,421
Derivative Assets	1,205	2,089
Derivative Liabilities	905	774
Gains (Losses)	(1,338)	(253)
Option swap [Member]
Derivative [Line Items]
Notional Value	0	24
Derivative Assets	0	9
Derivative Liabilities	0	2
Gains (Losses)	(70)	11
Total Return Swap [Member]
Derivative [Line Items]
Notional Value	90,043	146,001
Derivative Assets	503	1,402
Derivative Liabilities	1,044	972
Gains (Losses)	$ (8,766)	$ (160)