Investments (Details) - Fixed Maturities - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment disclosure details
Fixed maturities, amortized cost	$ 63,601	$ 61,316
Gross unrealized gains	712	1,556
Gross unrealized losses	849	178
Fixed maturities, at fair value	63,464	62,694
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,076	2,080
Gross unrealized gains	4	4
Gross unrealized losses	16	8
Fixed maturities, at fair value	2,064	2,076
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,329	29,996
Gross unrealized gains	489	968
Gross unrealized losses	207	49
Fixed maturities, at fair value	28,611	30,915
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	14,473	13,488
Gross unrealized gains	219	444
Gross unrealized losses	120	26
Fixed maturities, at fair value	14,572	13,906
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,755	11,307
Gross unrealized gains	172	338
Gross unrealized losses	74	19
Fixed maturities, at fair value	9,853	11,626
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,329	1,443
Gross unrealized gains	18	44
Gross unrealized losses	13	3
Fixed maturities, at fair value	1,334	1,484
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,772	3,758
Gross unrealized gains	80	142
Gross unrealized losses		1
Fixed maturities, at fair value	2,852	3,899
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,255	1,505
Gross unrealized gains	7	14
Gross unrealized losses	5	10
Fixed maturities, at fair value	1,257	1,509
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,557	2,334
Gross unrealized gains	54	87
Gross unrealized losses	38	11
Fixed maturities, at fair value	2,573	2,410
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	29,307	25,311
Gross unrealized gains	156	478
Gross unrealized losses	583	100
Fixed maturities, at fair value	28,880	25,689
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	77	90
Gross unrealized gains	2	5
Fixed maturities, at fair value	$ 79	$ 95