Dividend restrictions - Insurance subsidiaries (Detail) - USD ($) $ in Billions	Dec. 31, 2021	Dec. 31, 2020
Statutory Accounting Practices [Line Items]
Investments in fixed maturity and equity securities and short-term investments on deposit with U.S. state insurance authorities	$ 6.4	$ 5.5
Principal Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Amount available for payment of dividends without prior regulatory approval during 2022	30.0
United States Based Property and Casualty Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' equity	$ 301.0	$ 237.0

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=123600598&loc=SL75763979-209797

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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Amount of dividends available for payment after computation of statutory restrictions on capital surplus.

+ References

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The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

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