Stock-Based Compensation Plans - Restricted Stock and Performance Share Activity (Detail) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Restricted Shares, Beginning balance	383,978	432,138	435,989
Number of Restricted Shares, Granted	61,434	95,419	117,668
Number of Restricted Shares, Performance share pay-out	21,422	61,936	117,647
Number of Restricted Shares, Canceled	(4,590)	(12,786)	(3,845)
Number of Restricted Shares, Vested	(142,645)	(192,729)	(235,321)
Number of Restricted Shares, Ending balance	319,599	383,978	432,138
Weighted Average Grant Date Fair Value, Beginning balance	$ 71.50	$ 56.24	$ 41.83
Weighted Average Grant Date Fair Value, Ending balance	$ 88.77	$ 71.50	$ 56.24