Claim and Claim Adjustment Expense Reserves - Schedule of Impact of Loss Portfolio Transfer on the Consolidated Statement of Operations (Detail) - CNA Financial [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Claim And Claim Adjustment Expense Reserves [Line Items]
Net A&EP adverse development before consideration of LPT			$ 60	$ 200
Retroactive reinsurance benefit recognized	$ (17)	$ (12)	$ (60)	(94)
Pretax impact of A&EP reserve development and the LPT	$ (17)	$ (12)		$ 106

Net Asbestos And Environmental Pollution Adverse Development Before Consideration Of Loss Portfolio Transfer

No definition available.

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Namespace Prefix:	l_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration