Stock-Based Compensation - Time-Based Restricted Stock and Restricted Stock Unit Activity (Detail) - Time-Based Restricted Stock and Time-Based Restricted Stock Units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Restricted Stock Activity [Line Items]
Nonvested, Shares, Beginning Balance	121,800	107,907	329,562
Time-based Restricted Stock, Granted	36,201	57,874	57,598
Time-based Restricted Stock, Vested	(81,186)	(43,848)	(230,704)
Time-based Restricted Stock, Forfeited	(256)	(133)	(48,549)
Nonvested, Shares, Ending Balance	76,559	121,800	107,907
Nonvested, Weighted-Average Grant Price Per Share, Beginning Balance	$ 31.59	$ 29.43	$ 18.89
Weighted-Average Grant Price Per Share, Granted	72.54	37.64	43.81
Weighted-Average Grant Price Per Share, Vested	28.90	42.34	42.37
Weighted-Average Grant Price Per Share, Forfeited	37.36	43.89	20.20
Nonvested, Weighted-Average Grant Price Per Share, Ending Balance	$ 53.79	$ 31.59	$ 29.43