Summary of Nonvested Restricted Shares Activity (Detail) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
GRANT DATE FAIR VALUE
Nonvested awards outstanding, beginning balance | $ / shares	$ 55.27	[1]
Granted | $ / shares	67.48	[1]
Vested | $ / shares	59.24	[1]
Nonvested awards outstanding, ending balance | $ / shares	$ 60.16	[1]
NONVESTED NUMBER OF SHARES
Nonvested awards outstanding, beginning balance | shares	198,400
Granted | shares	119,200
Vested | shares	(105,900)
Nonvested awards outstanding, ending balance | shares	211,700

[1] Weighted Average