Equity (Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 70,975
Income tax benefit (expense)	(43,372)	$ (3,322)	$ 61,322
Ending balance	165,346	70,975
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(402)	(397)	(67)
Change in OCI before reclassifications	259	(8)	(507)
Amounts reclassified from AOCI	0	0	0
Income tax benefit (expense)	(91)	3	177
Ending balance	(234)	(402)	(397)
Net Unrealized Investment Gains (Losses)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	71,377	65,202	178,758
Change in OCI before reclassifications	164,482	74,040	(164,799)
Amounts reclassified from AOCI	(26,998)	(64,540)	(9,902)
Income tax benefit (expense)	(43,281)	(3,325)	61,145
Ending balance	165,580	71,377	65,202
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	70,975	64,805	178,691
Change in OCI before reclassifications	164,741	74,032	(165,306)
Amounts reclassified from AOCI	(26,998)	(64,540)	(9,902)
Income tax benefit (expense)	(43,372)	(3,322)	61,322
Ending balance	165,346	70,975	64,805
Cash flow hedges | Net Unrealized Investment Gains (Losses)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	41,000	48,000
Ending balance	$ (18,000)	$ 41,000	$ 48,000