Certain Long-Duration Contracts with Guarantees Liabilities for Guarantee Benefits (Details) - Annuity Contracts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	$ 8,382,545	$ 3,413,818	$ 3,387,128
Incurred guarantee benefits	462,488	(1,939,713)	60,217
Paid guarantee benefits	(60,851)	(57,813)	(29,751)
Change in unrealized investment gains and losses	13,048	(5,415)	(3,776)
Assumed guarantees upon reinsurance agreement with Pruco Life		6,971,668
Ending balance	8,797,230	8,382,545	3,413,818
GMDB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	637,319	265,877	255,613
Incurred guarantee benefits	29,605	43,185	43,167
Paid guarantee benefits	(57,053)	(55,604)	(29,240)
Change in unrealized investment gains and losses	12,931	(5,206)	(3,663)
Assumed guarantees upon reinsurance agreement with Pruco Life		389,067
Ending balance	622,802	637,319	265,877
GMAB/GMWB/ GMIWB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	7,707,333	3,134,077	3,112,411
Incurred guarantee benefits	444,569	(1,979,215)	21,666
Paid guarantee benefits	0	0	0
Change in unrealized investment gains and losses	0	0	0
Assumed guarantees upon reinsurance agreement with Pruco Life		6,552,471
Ending balance	8,151,902	7,707,333	3,134,077
GMIB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	37,893	13,864	19,104
Incurred guarantee benefits	(11,686)	(3,683)	(4,616)
Paid guarantee benefits	(3,798)	(2,209)	(511)
Change in unrealized investment gains and losses	117	(209)	(113)
Assumed guarantees upon reinsurance agreement with Pruco Life		30,130
Ending balance	$ 22,526	$ 37,893	$ 13,864