Employee Benefit Plans (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Y	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Pension and Other Postretirement Benefits Disclosure [Abstract]
Minimum Eligible Age For Employees To Contribute To Benefit Plan | Y		21
Employer Match Amount	$ 0.440	$ 0.375	$ 0.375	$ 0.375
Employee Contribution Amount	$ 1.00	$ 1.00	$ 1.00	$ 1.00
Maximum Percentage of Employee Salary Eligible for Employer Match	6.00%	6.00%	6.00%	6.00%
401 (k) Expense Recognized by Company		$ 6,900,000	$ 6,100,000	$ 6,200,000