Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Investment disclosure details
Fixed maturities, amortized cost	$ 60,482		$ 60,482		$ 59,650
Gross unrealized gains	1,670		1,670		1,369
Gross unrealized losses	245		245		504
Fixed maturities, at fair value	61,907		61,907		60,515
Equity securities, cost	558		558		504
Gross unrealized gains	153		153		236
Gross unrealized losses	11		11		8
Equity securities, at fair value	700		700		732
Continuous unrealized loss position less than 12 months, fair value	13,462		13,462		18,310
Continuous unrealized loss position less than 12 months, gross unrealized losses	226		226		482
Continuous unrealized loss position 12 months or longer, fair value	575		575		561
Continuous unrealized loss position 12 months or longer, gross unrealized losses	30		30		30
Continuous unrealized loss position, total, fair value	14,037		14,037		18,871
Continuous unrealized loss position, total, gross unrealized losses	256		256		512
Unrealized losses for available-for-sale securities for which fair value is less than 80% of amortized cost, total	$ 1		$ 1
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of the combined fixed maturity and equity security portfolios on a pre-tax basis (less than)	1.00%		1.00%
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%		1.00%
Impairment charges	$ 5	$ 4	$ 7	$ 22
Fixed maturities [Member]
Investment disclosure details
Gross realized gains			17	46
Gross realized losses			4	8
Continuous unrealized loss position less than 12 months, fair value	13,312		13,312		18,263
Continuous unrealized loss position less than 12 months, gross unrealized losses	223		223		480
Continuous unrealized loss position 12 months or longer, fair value	500		500		492
Continuous unrealized loss position 12 months or longer, gross unrealized losses	22		22		24
Continuous unrealized loss position, total, fair value	13,812		13,812		18,755
Continuous unrealized loss position, total, gross unrealized losses	245		245		504
Cumulative OTTI credit losses recognized for securities held	$ 83	$ 88	$ 83	$ 88
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of the fixed maturity portfolio on a pre-tax basis (less than)	1.00%	1.00%	1.00%	1.00%
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%	1.00%	1.00%	1.00%
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	$ 2,023		$ 2,023		2,031
Gross unrealized gains	10		10		9
Gross unrealized losses	5		5		5
Fixed maturities, at fair value	2,028		2,028		2,035
Continuous unrealized loss position less than 12 months, fair value	1,342		1,342		1,124
Continuous unrealized loss position less than 12 months, gross unrealized losses	5		5		5
Continuous unrealized loss position 12 months or longer, fair value	5		5
Continuous unrealized loss position, total, fair value	1,347		1,347		1,124
Continuous unrealized loss position, total, gross unrealized losses	5		5		5
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	31,323		31,323		31,550
Gross unrealized gains	944		944		712
Gross unrealized losses	153		153		352
Fixed maturities, at fair value	32,114		32,114		31,910
Continuous unrealized loss position less than 12 months, fair value	6,088		6,088		9,781
Continuous unrealized loss position less than 12 months, gross unrealized losses	142		142		352
Continuous unrealized loss position 12 months or longer, fair value	179		179		12
Continuous unrealized loss position 12 months or longer, gross unrealized losses	11		11
Continuous unrealized loss position, total, fair value	6,267		6,267		9,793
Continuous unrealized loss position, total, gross unrealized losses	153		153		352
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	14,010		14,010		13,955
Gross unrealized gains	410		410		271
Gross unrealized losses	79		79		182
Fixed maturities, at fair value	14,341		14,341		14,044
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	11,589		11,589		10,910
Gross unrealized gains	317		317		215
Gross unrealized losses	63		63		147
Fixed maturities, at fair value	11,843		11,843		10,978
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,612		1,612		1,717
Gross unrealized gains	45		45		36
Gross unrealized losses	11		11		22
Fixed maturities, at fair value	1,646		1,646		1,731
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	4,112		4,112		4,968
Gross unrealized gains	172		172		190
Gross unrealized losses					1
Fixed maturities, at fair value	4,284		4,284		5,157
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,537		1,537		1,631
Gross unrealized gains	24		24		34
Gross unrealized losses	5		5		3
Fixed maturities, at fair value	1,556		1,556		1,662
Continuous unrealized loss position less than 12 months, fair value	432		432		360
Continuous unrealized loss position less than 12 months, gross unrealized losses	5		5		3
Continuous unrealized loss position, total, fair value	432		432		360
Continuous unrealized loss position, total, gross unrealized losses	5		5		3
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,675		1,675		1,614
Gross unrealized gains	97		97		100
Gross unrealized losses	5		5		6
Fixed maturities, at fair value	1,767		1,767		1,708
Continuous unrealized loss position less than 12 months, fair value	470		470		528
Continuous unrealized loss position less than 12 months, gross unrealized losses	5		5		5
Continuous unrealized loss position 12 months or longer, fair value	37		37		43
Continuous unrealized loss position 12 months or longer, gross unrealized losses					1
Continuous unrealized loss position, total, fair value	507		507		571
Continuous unrealized loss position, total, gross unrealized losses	5		5		6
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	23,835		23,835		22,737
Gross unrealized gains	589		589		508
Gross unrealized losses	77		77		138
Fixed maturities, at fair value	24,347		24,347		23,107
Continuous unrealized loss position less than 12 months, fair value	4,980		4,980		6,470
Continuous unrealized loss position less than 12 months, gross unrealized losses	66		66		115
Continuous unrealized loss position 12 months or longer, fair value	279		279		437
Continuous unrealized loss position 12 months or longer, gross unrealized losses	11		11		23
Continuous unrealized loss position, total, fair value	5,259		5,259		6,907
Continuous unrealized loss position, total, gross unrealized losses	77		77		138
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	89		89		87
Gross unrealized gains	6		6		6
Fixed maturities, at fair value	95		95		93
Equity securities [Member]
Investment disclosure details
Gross realized gains			88	$ 8
Gross realized losses			1	$ 2
Continuous unrealized loss position less than 12 months, fair value	150		150		47
Continuous unrealized loss position less than 12 months, gross unrealized losses	3		3		2
Continuous unrealized loss position 12 months or longer, fair value	75		75		69
Continuous unrealized loss position 12 months or longer, gross unrealized losses	8		8		6
Continuous unrealized loss position, total, fair value	225		225		116
Continuous unrealized loss position, total, gross unrealized losses	11		11		8
Public common stock [Member]
Investment disclosure details
Equity securities, cost	443		443		390
Gross unrealized gains	130		130		216
Gross unrealized losses	6		6		3
Equity securities, at fair value	567		567		603
Continuous unrealized loss position less than 12 months, fair value	150		150		45
Continuous unrealized loss position less than 12 months, gross unrealized losses	3		3		2
Continuous unrealized loss position 12 months or longer, fair value	14		14		10
Continuous unrealized loss position 12 months or longer, gross unrealized losses	3		3		1
Continuous unrealized loss position, total, fair value	164		164		55
Continuous unrealized loss position, total, gross unrealized losses	6		6		3
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	115		115		114
Gross unrealized gains	23		23		20
Gross unrealized losses	5		5		5
Equity securities, at fair value	133		133		129
Continuous unrealized loss position less than 12 months, fair value					2
Continuous unrealized loss position 12 months or longer, fair value	61		61		59
Continuous unrealized loss position 12 months or longer, gross unrealized losses	5		5		5
Continuous unrealized loss position, total, fair value	61		61		61
Continuous unrealized loss position, total, gross unrealized losses	$ 5		$ 5		$ 5