Investments - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	$ 1,276	$ 1,190	$ 1,196
Less: investment expenses	50	43	43
Net investment income	1,226	1,147	1,153
Net investment income for separate accounts	225	236	262
Non-income producing real estate investments and securities partnerships, carrying value	191	220
Fixed Maturities [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	946	899	879
Equity securities [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	14	4	3
Commercial Mortgage Loans [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	81	91	112
Policy Loans [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	69	72	72
Other Long Term Investments [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	124	98	116
Short-term investments and cash [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Gross investment income	$ 42	$ 26	$ 14