Investments (Details) - Investment Information, Debt Securities - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investment disclosure details
Fixed maturities, amortized cost	$ 64,216		$ 63,601
Gross unrealized gains	1,488		712
Gross unrealized losses	204		849
Fixed maturities, at fair value	65,500		63,464
Proceeds from sales of fixed maturities classified as available for sale	769	$ 1,085
Gross realized gains	21	6
Gross realized losses	4	$ 6
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,029		2,076
Gross unrealized gains	7		4
Gross unrealized losses	6		16
Fixed maturities, at fair value	2,030		2,064
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,451		28,329
Gross unrealized gains	920		489
Gross unrealized losses	24		207
Fixed maturities, at fair value	29,347		28,611
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	14,879		14,473
Gross unrealized gains	468		219
Gross unrealized losses	14		120
Fixed maturities, at fair value	15,333		14,572
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,757		9,755
Gross unrealized gains	336		172
Gross unrealized losses	8		74
Fixed maturities, at fair value	10,085		9,853
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,300		1,329
Gross unrealized gains	37		18
Gross unrealized losses	2		13
Fixed maturities, at fair value	1,335		1,334
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,515		2,772
Gross unrealized gains	79		80
Fixed maturities, at fair value	2,594		2,852
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,184		1,255
Gross unrealized gains	12		7
Gross unrealized losses	2		5
Fixed maturities, at fair value	1,194		1,257
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,601		2,557
Gross unrealized gains	64		54
Gross unrealized losses	16		38
Fixed maturities, at fair value	2,649		2,573
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	29,873		29,307
Gross unrealized gains	482		156
Gross unrealized losses	156		583
Fixed maturities, at fair value	30,199		28,880
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	78		77
Gross unrealized gains	3		2
Fixed maturities, at fair value	$ 81		$ 79

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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