Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 4,633	$ 4,842	$ 4,789	$ 4,691	$ 4,587	$ 4,192	$ 4,214	$ 4,169	$ 18,955	$ 17,162	$ 16,291
Benefits, losses and expenses	4,466	4,312	4,252	4,172	4,165	4,011	4,495	3,768	17,202	16,439	15,844
Income (loss) from continuing operations, net of tax	196	427	434	428	(558)	145	(152)	303	1,485	(262)	613
Income (loss) from discontinued operations, net of tax	0	5	148	169	(3,145)	89	112	75
Net income (loss)	196	432	582	597	(3,703)	234	(40)	378	1,807	(3,131)	896
Preferred stock dividends	6	0	0	0	0	0	0	0	6	0	0
Net income (loss) available to common stockholders	$ 190	$ 432	$ 582	$ 597	$ (3,703)	$ 234	$ (40)	$ 378	$ 1,801	$ (3,131)	$ 896
Basic
Income (loss) from continuing operations, net of tax, available to common stockholders	$ 0.53	$ 1.19	$ 1.21	$ 1.20	$ (1.56)	$ 0.40	$ (0.42)	$ 0.82	$ 4.13	$ (0.72)	$ 1.58
(Loss) income from discontinued operations, net of tax (in USD per share)	0	0.01	0.41	0.47	(8.81)	0.25	0.31	0.20	0.90	(7.89)	0.73
Net income (loss) available to common stockholders	0.53	1.20	1.62	1.67	(10.37)	0.65	(0.11)	1.02	5.03	(8.61)	2.31
Diluted
Income (loss) from continuing operations, net of tax, available to common stockholders	0.52	1.17	1.19	1.18	(1.56)	0.40	(0.42)	0.80	4.06	(0.72)	1.55
(Loss) income from discontinued operations, net of tax (in USD per share)	0	0.02	0.41	0.46	(8.81)	0.24	0.31	0.20	0.89	(7.89)	0.72
Net income (loss) available to common stockholders	$ 0.52	$ 1.19	$ 1.60	$ 1.64	$ (10.37)	$ 0.64	$ (0.11)	$ 1.00	$ 4.95	$ (8.61)	$ 2.27
Weighted average common shares outstanding, basic	359.1	358.6	358.3	357.5	357.0	360.2	366.0	371.4	358.4	363.7	387.7
Weighted average shares outstanding and dilutive potential common shares (in shares)	364.0	364.1	364.2	363.9	357.0	367.0	366.0	378.6	364.1	363.7	394.8