General (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized gains (losses) on investments	$ (438)	$ 63
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax				$ (50)	$ 0
ASU 2016-01 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax			$ 28
ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax	12
Net unrealized gains (losses) on investments	142
Decrease in unrecognized pension and postretirement benefits	$ 130
Loews | CNAF Consolidated
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noncontrolling interest, ownership percentage by parent	89.00%
Preferred stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)	$ (15)
Change in fair value of equity securities	5
Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)	1
Common Stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)	1
Change in fair value of equity securities	$ (1)