Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 5,361	$ 5,347	$ 5,092	$ 4,940	$ 4,633	$ 4,842	$ 4,789	$ 4,691	$ 20,740	$ 18,955	$ 17,162
Benefits, losses and expenses	4,685	4,694	4,636	4,165	4,466	4,312	4,252	4,172	18,180	17,202	16,439
Income from continuing operations, net of tax	548	535	372	630	196	427	434	428	2,085	1,485	(262)
Income from discontinued operations, net of tax	0	0	0	0	0	5	148	169	0	322	(2,869)
Net income (loss)	548	535	372	630	196	432	582	597	2,085	1,807	(3,131)
Preferred stock dividends	5	11	0	5	6	0	0	0	21	6	0
Net income (loss) available to common stockholders	$ 543	$ 524	$ 372	$ 625	$ 190	$ 432	$ 582	$ 597	$ 2,064	$ 1,801	$ (3,131)
Basic
Income (loss) from continuing operations, net of tax, available to common stockholders	$ 1.51	$ 1.45	$ 1.03	$ 1.74	$ 0.53	$ 1.19	$ 1.21	$ 1.20	$ 5.72	$ 4.13	$ (0.72)
(Loss) income from discontinued operations, net of tax (in USD per share)	0	0	0	0	0	0.01	0.41	0.47	0	0.90	(7.89)
Net income (loss) available to common stockholders	1.51	1.45	1.03	1.74	0.53	1.20	1.62	1.67	5.72	5.03	(8.61)
Diluted
Income (loss) from continuing operations, net of tax, available to common stockholders	1.49	1.43	1.02	1.71	0.52	1.17	1.19	1.18	5.66	4.06	(0.72)
(Loss) income from discontinued operations, net of tax (in USD per share)	0	0	0	0	0	0.02	0.41	0.46	0	0.89	(7.89)
Net income (loss) available to common stockholders	$ 1.49	$ 1.43	$ 1.02	$ 1.71	$ 0.52	$ 1.19	$ 1.60	$ 1.64	$ 5.66	$ 4.95	$ (8.61)