Incentive Plan - Summary of Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Shares, Outstanding at beginning of year	2,758,833	2,281,931	1,751,270
Number of Shares, Granted	335,150	317,460	238,305
Number of Shares, Assumed in acquisition	0	886,968
Number of Shares, Exercised	(567,153)	(375,576)	(29,480)
Number of Shares, Forfeited	(35,099)	(11,625)	(8,900)
Number of Shares, Expired	(2,417)	(340,325)	(39,264)
Number of Shares, Outstanding at end of year	2,489,314	2,758,833	2,281,931
Number of Shares, Options exercisable at end of year	1,608,762	1,912,630	1,497,960
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted Average Exercise Price, Outstanding at beginning of year (in dollars per share)	$ 14.94	$ 17.62	$ 15.94
Weighted Average Exercise Price, Granted (in dollars per share)	29.01	17.27	17.39
Weighted Average Exercise Price, Assumed in acquisition	0.00	10.30
Weighted Average Exercise Price, Exercised (in dollars per share)	14.39	13.20	13.43
Weighted Average Exercise Price, Forfeited (in dollars per share)	18.42	16.37	14.56
Weighted Average Exercise Price, Expired (in dollars per share)	11.70	25.02	25.31
Weighted Average Exercise Price, Outstanding at end of year (in dollars per share)	$ 16.91	$ 14.94	$ 17.62