Stock-based Compensation Plans - PG/RG-PSUs Activities (Parenthetical) (Detail) - Performance-based PG-PSUs and RG-PSUs - Tranche	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total Number of tranches in PSU		3
Number of tranches forfeited		1
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%