Stock-based Compensation Plans - TSR - PSUs Activities (Parenthetical) (Detail) - Performance Based TSR - PSUs	3 Months Ended
	Mar. 31, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested and issued related to performance period number of common stock	20,430	[1]
Return on common stock percentile	0.00%
February 2015 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile	46.00%
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	200.00%

[1] Includes 20,430 TSR-PSUs awards issued related to the 2015 performance period where the return on our common stock was the 46th percentile for the February 2015 grant. The remaining 2015 awards are considered to be forfeited.