Stock-based Compensation Plans - PG/RG-PSUs Activities (Detail) - Performance-based PG-PSUs and RG-PSUs - $ / shares		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Beginning Balance		122,921
Granted		440,938	[1]	122,921
Outstanding, Ending Balance		536,798		122,921
Forfeited	[2]	(27,061)
Outstanding, Beginning Balance, weighted average grant date fair value		$ 25.53
Granted, weighted average grant date fair value		15.22	[1]	$ 25.53
Outstanding, Ending Balance, weighted average grant date fair value		15.61		$ 25.53
Forfeited, weighted average grant date fair value	[2]	$ 23.03

[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero and 200% depending on achievement of specifically identified performance targets.

[2] The first of three tranches of PG-PSUs granted in 2017 was forfeited as the performance metric was not met.