Stock-based Compensation Plans - PG/RG-PSUs Activities (Detail) - Performance-based PG-PSUs and RG-PSUs	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Beginning Balance | shares	0
Granted | shares	122,921	[1]
Outstanding, Ending Balance | shares	122,921
Outstanding, Beginning Balance, weighted average grant date fair value | $ / shares	$ 0
Granted, weighted average grant date fair value | $ / shares	25.53	[1]
Outstanding, Ending Balance, weighted average grant date fair value | $ / shares	$ 25.53

[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero and 150% depending on achievement of specifically identified performance targets.