Stock-Based Compensation Plans - Restricted Stock and Performance Share Activity (Detail) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Restricted Shares, Beginning balance	319,599	383,978	432,138
Number of Restricted Shares, Granted	59,179	61,434	95,419
Number of Restricted Shares, Performance share pay-out	8,094	21,422	61,936
Number of Restricted Shares, Canceled	(4,046)	(4,590)	(12,786)
Number of Restricted Shares, Vested	(110,956)	(142,645)	(192,729)
Number of Restricted Shares, Ending balance	271,870	319,599	383,978
Weighted Average Grant Date Fair Value, Beginning balance	$ 88.77	$ 71.50	$ 56.24
Weighted Average Grant Date Fair Value, Ending balance	$ 112.47	$ 88.77	$ 71.50