Shareholders' Equity - Reconciliation of Outstanding Balance of Stock Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options, Outstanding at beginning	421,000	490,000	601,000
Options, Granted	200,000	0	0
Options, Exercised	(106,000)	(69,000)	(111,000)
Options, Forfeited or expired	0	0	0
Options, Outstanding at ending	515,000	421,000	490,000
Options, Exercisable at ending	331,000
Options, Unvested at ending	184,000
Weighted Average Exercise Price, Outstanding at beginning	$ 19.06	$ 17.73	$ 16.97
Weighted Average Exercise Price, Granted	30.50	0	0
Weighted Average Exercise Price, Exercised	11.17	9.64	13.61
Weighted Average Exercise Price, Forfeited or expired	0	0	0
Weighted Average Exercise Price, Outstanding at ending	25.13	19.06	17.73
Weighted Average Exercise Price, Exercisable at ending	22.16
Weighted Average Exercise Price, Unvested at ending	30.50
Weighted Average Grant Date Fair Value, Outstanding at beginning	9.01	8.69	8.42
Weighted Average Grant Date Fair Value, Granted	14.58	0	0
Weighted Average Grant Date Fair Value, Exercised	6.75	6.75	7.25
Weighted Average Grant Date Fair Value, Forfeited or expired	0	0	0
Weighted Average Grant Date Fair Value, Outstanding at ending	$ 11.64	$ 9.01	$ 8.69
Aggregate Intrinsic Value, Outstanding	$ 5,670
Aggregate Intrinsic Value, Exercisable	4,634
Aggregate Intrinsic Value, Unvested	$ 1,036