Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit Plans [Line Items]
Percentage of defined plan agreement	30.00%
Contributions to plan	$ 0.9	$ 1.0	$ 1.1
Supplemental Retirement Plans expenses	$ 0.3	$ 0.4	$ 0.4
Employee Share Purchase Plan [Member]
Benefit Plans [Line Items]
Common stock purchase at a discount	15.00%
Number of common shares purchased	38,000	24,000	25,000
Shares purchase consideration	$ 0.5	$ 0.5	$ 0.5
Expenses recorded to the share purchase plan	$ 0.1	$ 0.1	$ 0.1