Stock-based Compensation Plans - PSUA Activities (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2014
Market-based TSR
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years	3 years
Minimum
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	150.00%