Dividend restrictions - Insurance subsidiaries (Detail) - USD ($) $ in Billions	Dec. 31, 2020	Dec. 31, 2019
Statutory Accounting Practices [Line Items]
Investments in fixed maturity and equity securities and short-term investments on deposit with U.S. state insurance authorities	$ 5.5	$ 6.3
Principal Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Amount available for payment of dividends without prior regulatory approval during 2021	23.0
United States Based Property and Casualty Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' equity	$ 237.0	$ 216.0

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=75763970&loc=SL75763979-209797

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+ References

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

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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
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