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, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Value	$ 475,976	$ 459,743
Derivative Assets	11,619	8,413
Derivative Liabilities	14,323	8,649
Gains (Losses)	(23,861)	(15,528)
Exchange-Traded Future [Member]
Derivative [Line Items]
Notional Value	163,458	103,108
Derivative Assets	948	1,224
Derivative Liabilities	2,540	1,092
Gains (Losses)	(15,343)	(2,754)
Currency Forwards [Member]
Derivative [Line Items]
Notional Value	126,503	180,820
Derivative Assets	8,306	4,541
Derivative Liabilities	8,058	4,711
Gains (Losses)	(457)	(2,028)
Interest Rate Swap [Member]
Derivative [Line Items]
Notional Value	43,309	40,664
Derivative Assets	951	940
Derivative Liabilities	870	897
Gains (Losses)	(137)	(572)
Credit Default Swap [Member]
Derivative [Line Items]
Notional Value	74,600	45,108
Derivative Assets	1,247	1,205
Derivative Liabilities	2,465	905
Gains (Losses)	(1,757)	(1,338)
Option Swap [Member]
Derivative [Line Items]
Notional Value		0
Derivative Assets		0
Derivative Liabilities		0
Gains (Losses)		(70)
Total Return Swap [Member]
Derivative [Line Items]
Notional Value	68,106	90,043
Derivative Assets	167	503
Derivative Liabilities	390	1,044
Gains (Losses)	$ (6,167)	$ (8,766)