Employee Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Shares held by the Rabbi trust	170,310	166,807
Performance incentive accruals	$ 12.2	$ 5.1	$ 2.9
Shares purchased by custodians	8,063	20,532	7,327
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Payroll deduction for ESPP, percent of annual wages	10.00%
Highly Compensated Employees [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Payroll deduction for ESPP, percent of annual wages	20.00%
401(k)Saving Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Company match of contribution	50.00%
Employees contribution share	6.00%
Contribution in operation	$ 8.3	$ 7.1	$ 6.4