Treasury Stock - Treasury Stock Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Treasury Stock [Roll Forward]
Beginning balance (shares)	7,664,325		5,648,778		6,788,817	4,499,655	4,499,655
Ending balance (shares)	7,740,925	7,664,325	6,589,817	5,648,778	7,740,925	6,589,817	6,788,817	4,499,655
Beginning balance	$ 167,919		$ 116,592		$ 143,357	$ 88,756	$ 88,756
Ending balance	$ 170,782	$ 167,919	$ 137,884	$ 116,592	$ 170,782	$ 137,884	$ 143,357	$ 88,756
Average share price (usd per share)	$ 22.06	$ 21.91	$ 20.92	$ 20.64	$ 22.06	$ 20.92	$ 21.12	$ 19.73
Repurchases on open market
Treasury Stock [Roll Forward]
Shares repurchased (shares)	76,600		941,039		952,108	2,090,162
Shares repurchased	$ 2,863		$ 21,292		$ 27,425	$ 49,128
Average share price (usd per share)	$ 37.38		$ 22.63		$ 28.80	$ 23.50