Long-Duration Insurance Contracts Targeted Improvements - Additional Information (Detail)	Jan. 01, 2021
Long Duration Insurance Contracts [Line Items]
Net premium ratio percentage limit for cohorts	100.00%

The limit of the net premium ratio for cohorts used in determining the retained earnings impact upon adoption of new accounting guidance.

No definition available.

Name:	gnw_NetPremiumRatioLimitForCohorts
Namespace Prefix:	gnw_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant