Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 4,539	$ 4,144	$ 4,168	$ 4,123	$ 3,978	$ 4,114	$ 4,085	$ 3,930	$ 16,974	$ 16,107	$ 15,997
Benefits, losses and expenses	4,117	3,963	4,449	3,722	4,354	3,690	4,060	3,556	16,251	15,660	14,519
Income (loss) from continuing operations, net of tax	(558)	145	(152)	303	(135)	351	99	298	(262)	613	1,189
Income from discontinued operations, net of tax	(3,145)	89	112	75	54	87	117	25	(2,869)	283	493
Net income (loss)	$ (3,703)	$ 234	$ (40)	$ 378	$ (81)	$ 438	$ 216	$ 323	$ (3,131)	$ 896	$ 1,682
Basic
(Loss) income from continuing operations, net of tax (in USD per share)	$ (1.56)	$ 0.40	$ (0.42)	$ 0.82	$ (0.36)	$ 0.91	$ 0.25	$ 0.75	$ (0.72)	$ 1.58	$ 2.86
(Loss) income from discontinued operations, net of tax (in USD per share)	(8.81)	0.25	0.31	0.20	0.14	0.23	0.30	0.06	(7.89)	0.73	1.19
Basic earnings (losses) per common share (in USD per share)	(10.37)	0.65	(0.11)	1.02	(0.22)	1.14	0.55	0.81	(8.61)	2.31	4.05
Diluted
(Loss) income from continuing operations, net of tax (in USD per share)	(1.56)	0.40	(0.42)	0.80	(0.36)	0.90	0.25	0.73	(0.72)	1.55	2.80
(Loss) income from discontinued operations, net of tax (in USD per share)	(8.81)	0.24	0.31	0.20	0.14	0.22	0.29	0.06	(7.89)	0.72	1.16
Diluted earnings (losses) per common share (in USD per share)	$ (10.37)	$ 0.64	$ (0.11)	$ 1.00	$ (0.22)	$ 1.12	$ 0.54	$ 0.79	$ (8.61)	$ 2.27	$ 3.96
Weighted average common shares outstanding, basic (in shares)	357.0	360.2	366.0	371.4	376.6	383.8	391.8	398.5	363.7	387.7	415.5
Weighted average shares outstanding and dilutive potential common shares (in shares)	357.0	367.0	366.0	378.6	376.6	390.5	398.6	406.3	363.7	394.8	425.2

The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=96948231&loc=d3e4984-109258

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1252-109256

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882