Certain Long-Duration Contracts with Guarantees Liabilities for Guarantee Benefits (Details) - Annuity Contracts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	$ 8,797,230	$ 8,382,545	$ 3,413,818
Incurred guarantee benefits	286,847	462,488	(1,939,713)
Paid guarantee benefits	(70,802)	(60,851)	(57,813)
Change in unrealized investment gains and losses	(20,338)	13,048	(5,415)
Assumed guarantees upon reinsurance agreement with Pruco Life			6,971,668
Ending balance	8,992,937	8,797,230	8,382,545
GMDB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	622,802	637,319	265,877
Incurred guarantee benefits	103,596	29,605	43,185
Paid guarantee benefits	(67,887)	(57,053)	(55,604)
Change in unrealized investment gains and losses	(20,108)	12,931	(5,206)
Assumed guarantees upon reinsurance agreement with Pruco Life			389,067
Ending balance	638,403	622,802	637,319
GMAB/GMWB/ GMIWB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	8,151,902	7,707,333	3,134,077
Incurred guarantee benefits	180,572	444,569	(1,979,215)
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	8,332,474	8,151,902	7,707,333
GMIB
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	22,526	37,893	13,864
Incurred guarantee benefits	2,679	(11,686)	(3,683)
Paid guarantee benefits	(2,915)	(3,798)	(2,209)
Change in unrealized investment gains and losses	(230)	117	(209)
Assumed guarantees upon reinsurance agreement with Pruco Life			30,130
Ending balance	$ 22,060	$ 22,526	$ 37,893