Investments (Details) - Investment Information, Debt Securities - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment disclosure details
Fixed maturities, amortized cost	$ 64,784		$ 63,601
Gross unrealized gains	2,428		712
Gross unrealized losses	40		849
Fixed maturities, at fair value	67,172		63,464
Proceeds from sales of fixed maturities classified as available for sale	1,495	$ 2,607
Gross realized gains	35	24
Gross realized losses	4	$ 11
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,000		2,076
Gross unrealized gains	20		4
Gross unrealized losses	2		16
Fixed maturities, at fair value	2,018		2,064
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,412		28,329
Gross unrealized gains	1,319		489
Gross unrealized losses	1		207
Fixed maturities, at fair value	29,730		28,611
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	15,187		14,473
Gross unrealized gains	692		219
Gross unrealized losses	1		120
Fixed maturities, at fair value	15,878		14,572
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,701		9,755
Gross unrealized gains	499		172
Gross unrealized losses	0		74
Fixed maturities, at fair value	10,200		9,853
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,273		1,329
Gross unrealized gains	53		18
Gross unrealized losses	0		13
Fixed maturities, at fair value	1,326		1,334
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,251		2,772
Gross unrealized gains	75		80
Gross unrealized losses	0		0
Fixed maturities, at fair value	2,326		2,852
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,112		1,255
Gross unrealized gains	14		7
Gross unrealized losses	1		5
Fixed maturities, at fair value	1,125		1,257
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,839		2,557
Gross unrealized gains	90		54
Gross unrealized losses	4		38
Fixed maturities, at fair value	2,925		2,573
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	30,371		29,307
Gross unrealized gains	982		156
Gross unrealized losses	32		583
Fixed maturities, at fair value	31,321		28,880
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	50		77
Gross unrealized gains	3		2
Gross unrealized losses	0		0
Fixed maturities, at fair value	$ 53		$ 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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