Summary of Common Shares (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Authorized	1,100,000,000	1,100,000,000
Common shares outstanding	783,000,000	831,000,000	847,599,000	847,599,000