Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - Debt Securities - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 10,734	$ 10,643
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	320	174
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	32	128
Fair Value	11,022	10,689
Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,753	5,712
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	160	85
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	25	87
Fair Value	5,888	5,710
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,269	4,251
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	153	84
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	31
Fair Value	4,419	4,304
Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	305	316
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	7
Fair Value	302	310
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	292	287
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	6	3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1	2
Fair Value	297	288
United States government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	99	67
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	1
Fair Value	100	67
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	16	10
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	$ 16	$ 10

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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