Stock-based Compensation Plans - TSR - PSUs Activities (Parenthetical) (Detail) - Performance Based TSR - PSUs	9 Months Ended
	Sep. 30, 2020
May 2017 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile	80.00%
March 2017 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile	60.00%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares on performance units granted	0.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares on performance units granted	200.00%