Share Based Compensation (Assumptions Used in Monte Carlo Simulations Used to Determine Aggregate Fair Values) (Detail) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2014 USD ($) $ / shares	Jan. 29, 2018 USD ($) $ / shares
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Aggregate Fair Value Of Awards On Grant Date		$ 800	$ 700	$ 800
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.42%	2.19%	2.19%	0.90%
Aggregate Fair Value Of Awards On Grant Date		$ 1,779	$ 1,620	$ 1,914	$ 1,336
Aggregate Fair Value Per Share Based On Monte Carlo Simulation | $ / shares		$ 14.56	$ 11.53	$ 15.02	$ 10.93
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	25.80%	31.60%	31.60%	25.30%
Stock Beta Of Restricted Stock Unit Awards	0.706			1.184