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
	Mar. 31, 2026	Mar. 31, 2025
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	$ 2,539	$ 2,216
Reported in Net investment gains (losses)	$ (6)	$ 2
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	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 (loss) income	Other comprehensive (loss) income
Reported in Other comprehensive income (loss)	$ (41)	$ 24
Total realized and unrealized investment gains (losses)	(43)	29
Purchases	103	82
Sales	0	0
Settlements	(25)	(41)
Transfers into Level 3	0	15
Transfers out of Level 3	(49)	0
Balance, Ending, Assets	2,525	2,301
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	(1)	(1)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(41)	24
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	1,483	1,278
Reported in Net investment gains (losses)	(3)	0
Reported in Other comprehensive income (loss)	(24)	21
Total realized and unrealized investment gains (losses)	(27)	21
Purchases	63	55
Sales	0	0
Settlements	(5)	(18)
Transfers into Level 3	0	15
Transfers out of Level 3	0	0
Balance, Ending, Assets	1,514	1,351
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(24)	21
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	45	42
Reported in Net investment gains (losses)	0	0
Reported in Other comprehensive income (loss)	(1)	2
Total realized and unrealized investment gains (losses)	(1)	2
Purchases	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	44	44
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(1)	2
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	998	876
Reported in Net investment gains (losses)	(3)	0
Reported in Other comprehensive income (loss)	(16)	1
Total realized and unrealized investment gains (losses)	(14)	5
Purchases	40	27
Sales	0	0
Settlements	(20)	(19)
Transfers into Level 3	0	0
Transfers out of Level 3	(49)	0
Balance, Ending, Assets	955	889
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(16)	1
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	13	20
Reported in Net investment gains (losses)	0	2
Reported in Other comprehensive income (loss)	0	0
Total realized and unrealized investment gains (losses)	(1)	1
Purchases	0	0
Sales	0	0
Settlements	0	(4)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	12	17
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	(1)	(1)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0
Reported in Net investment income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	4	3
Reported in Net investment income \| Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	0	0
Reported in Net investment income \| 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
Reported in Net investment income \| Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	5	4
Reported in Net investment income \| Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	$ (1)	$ (1)

CNA FINANCIAL CORP 10-Q