Common Stock Repurchases - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchased and cancelled shares	15,130	38,970
Common stock repurchased, value	$ 1,100,000	$ 2,200,000
Common stock repurchased, average price	$ 75.28	$ 56.66
Share Repurchase Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchased and cancelled shares	782,425	430,866
Common stock repurchased, value	$ 59,000,000	$ 22,500,000
Common stock repurchased, average price	$ 75.41	$ 52.15
Share repurchase program shares authorized to be repurchased	$ 250,000,000