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Employee Benefit Plans (Deferred Compensation Plans) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Compensation_Program	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Compensation And Retirement Disclosure [Abstract]
Number of deferred compensation programs \| Compensation_Program	4
Maximum percentage of cash compensation allowed to be deferred under the deferred compensation plan	25.00%
Criteria for benefit distribution	Six months after termination of employment in a single lump sum payment or annual installments paid over a three or ten year term.
Deferred compensation income (expense) adjustments due to fair value of the trust assets \| $	$ (2.6)	$ (1.3)	$ 0.3