Reserve for Life-Contingent Contract Benefits and Contractholder Funds - Reserve for life-contingent contract benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 11,625	$ 11,322
Structured settlement annuities
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 6,994	6,681
Structured settlement annuities | Minimum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	2.90%
Structured settlement annuities | Maximum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	9.00%
Other immediate fixed annuities
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 1,850	1,935
Other immediate fixed annuities | Minimum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	0.00%
Other immediate fixed annuities | Maximum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	11.50%
Traditional life insurance
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 2,458	2,373
Traditional life insurance | Minimum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	2.50%
Traditional life insurance | Maximum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	11.30%
Accident and health insurance
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 238	232
Accident and health insurance | Minimum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	3.00%
Accident and health insurance | Maximum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	6.00%
Other
Reserve for life-contingent benefits:
Total reserve for life-contingent contract benefits	$ 85	$ 101
Other | Minimum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	2.00%
Other | Maximum
Key assumptions generally used in calculating the reserve for life-contingent contract benefits:
Life contingent contract benefits, interest rate assumptions (as a percent)	5.80%