Stock-based Compensation Plans - PG/RG-PSUs Activities (Parenthetical) (Detail) - Performance-based PG-PSUs and RG-PSUs	9 Months Ended
	Sep. 30, 2018 Tranche
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of tranches forfeited	1
Total Number of tranches in PSU	3
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares granted	0.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares granted	200.00%