Stock-based Compensation Plans - PG/RG-PSUs Activities (Detail) - Performance-based PG-PSUs and RG-PSUs	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Beginning Balance | shares	122,921
Granted | shares	440,938	[1]
Forfeited | shares	(20,488)	[2]
Outstanding, Ending Balance | shares	543,371
Outstanding, Beginning Balance, weighted average grant date fair value | $ / shares	$ 18.66
Granted, weighted average grant date fair value | $ / shares	15.22	[1]
Forfeited, weighted average grant date fair value | $ / shares	25.53	[2]
Outstanding, Ending Balance, weighted average grant date fair value | $ / shares	$ 15.61

[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 are considered forfeited as the performance metric was not met.