Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	$ 2,525	$ 2,301	$ 2,539	$ 2,216
Reported in Net investment gains (losses)	$ (2)	$ (3)	$ (8)	$ (1)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment losses	Net investment losses	Net investment losses	Net investment losses
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other comprehensive income (loss)	Other comprehensive income (loss)	Other comprehensive income (loss)	Other comprehensive income (loss)
Reported in Other comprehensive income (loss)	$ (3)	$ 12	$ (44)	$ 36
Total realized and unrealized investment gains (losses)	2	14	(41)	43
Purchases	143	66	246	148
Sales	(5)	(7)	(5)	(7)
Settlements	(49)	(46)	(74)	(87)
Transfers into Level 3	0	0	0	15
Transfers out of Level 3	(25)	0	(74)	0
Balance, Ending, Assets	2,591	2,328	2,591	2,328
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	(1)	(1)	(1)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(3)	12	(44)	36
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	1,514	1,351	1,483	1,278
Reported in Net investment gains (losses)	0	1	(3)	1
Reported in Other comprehensive income (loss)	2	16	(22)	37
Total realized and unrealized investment gains (losses)	2	17	(25)	38
Purchases	101	44	164	99
Sales	0	0	0	0
Settlements	(23)	(24)	(28)	(42)
Transfers into Level 3	0	0	0	15
Transfers out of Level 3	0	0	0	0
Balance, Ending, Assets	1,594	1,388	1,594	1,388
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	2	16	(22)	37
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	44	44	45	42
Reported in Net investment gains (losses)	0	0	0	0
Reported in Other comprehensive income (loss)	0	0	(1)	2
Total realized and unrealized investment gains (losses)	0	0	(1)	2
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, Ending, Assets	44	44	44	44
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0	(1)	2
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	955	889	998	876
Reported in Net investment gains (losses)	(3)	(4)	(6)	(4)
Reported in Other comprehensive income (loss)	(5)	(4)	(21)	(3)
Total realized and unrealized investment gains (losses)	(3)	(3)	(17)	2
Purchases	42	22	82	49
Sales	0	0	0	0
Settlements	(26)	(22)	(46)	(41)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(25)	0	(74)	0
Balance, Ending, Assets	943	886	943	886
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(5)	(4)	(21)	(3)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	12	17	13	20
Reported in Net investment gains (losses)	1	0	1	2
Reported in Other comprehensive income (loss)	0	0	0	0
Total realized and unrealized investment gains (losses)	3	0	2	1
Purchases	0	0	0	0
Sales	(5)	(7)	(5)	(7)
Settlements	0	0	0	(4)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, Ending, Assets	10	10	10	10
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	(1)	(1)	(1)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0	0	0
Location, Statement of Income, Balance [Axis]: cna:NetInvestmentIncomeAndGainsLossesOnTradingSecurities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	7	5	11	8
Location, Statement of Income, Balance [Axis]: cna:NetInvestmentIncomeAndGainsLossesOnTradingSecurities | Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	0	0	0	0
Location, Statement of Income, Balance [Axis]: cna:NetInvestmentIncomeAndGainsLossesOnTradingSecurities | States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	0	0	0	0
Location, Statement of Income, Balance [Axis]: cna:NetInvestmentIncomeAndGainsLossesOnTradingSecurities | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	5	5	10	9
Location, Statement of Income, Balance [Axis]: cna:NetInvestmentIncomeAndGainsLossesOnTradingSecurities | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	$ 2	$ 0	$ 1	$ (1)