Investments (Details) - Investment Information, Debt Securities - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investment disclosure details
Fixed maturities, amortized cost	$ 62,674		$ 61,316
Gross unrealized gains	742		1,556
Gross unrealized losses	880		178
Fixed maturities, at fair value	62,536		62,694
Proceeds from sales of fixed maturities classified as available for sale	2,607	$ 563
Gross realized gains	24	17
Gross realized losses	11	$ 4
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,041		2,080
Gross unrealized gains	1		4
Gross unrealized losses	23		8
Fixed maturities, at fair value	2,019		2,076
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,665		29,996
Gross unrealized gains	502		968
Gross unrealized losses	237		49
Fixed maturities, at fair value	28,930		30,915
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,992		13,488
Gross unrealized gains	201		444
Gross unrealized losses	136		26
Fixed maturities, at fair value	14,057		13,906
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,857		11,307
Gross unrealized gains	171		338
Gross unrealized losses	88		19
Fixed maturities, at fair value	9,940		11,626
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,384		1,443
Gross unrealized gains	19		44
Gross unrealized losses	12		3
Fixed maturities, at fair value	1,391		1,484
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,432		3,758
Gross unrealized gains	111		142
Gross unrealized losses	1		1
Fixed maturities, at fair value	3,542		3,899
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,281		1,505
Gross unrealized gains	8		14
Gross unrealized losses	8		10
Fixed maturities, at fair value	1,281		1,509
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,499		2,334
Gross unrealized gains	66		87
Gross unrealized losses	49		11
Fixed maturities, at fair value	2,516		2,410
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,102		25,311
Gross unrealized gains	161		478
Gross unrealized losses	563		100
Fixed maturities, at fair value	27,700		25,689
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	86		90
Gross unrealized gains	4		5
Fixed maturities, at fair value	$ 90		$ 95

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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