Stock-based Compensation Plans - TSR - PSUs Activities (Parenthetical) (Detail) - Performance Based TSR - PSUs - shares	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested and issued related to performance period number of common stock	73,985	[1]	85,461	[2]
February 2014 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile			67.00%
May 2014 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile			56.00%
February 2015 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile	46.00%
May 2015 Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Return on common stock percentile	36.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares on performance units granted	0.00%		0.00%		0.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Potential payout of shares on performance units granted	200.00%		150.00%		150.00%

[1] Includes 73,985 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 and the 36th percentile for the May 2015 grant. The remaining 2015 awards are considered to be forfeited.

[2] Includes 85,461 TSR-PSU awards issued related to the 2014 performance period where the return on our common stock was the 67th percentile for the February 2014 grant and 56th percentile for the May 2014 grant. The remaining 2014 awards are considered to be forfeited.