Employee Benefit Plans (401(k) Savings Plan) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Criteria of employer to contribute in employee saving plan	100% of an employee’s first 3% of contributions and 50% of their next 2% of contributions
Percentage of employer match to employee's contribution	100.00%
Percentage of employer match to employee's contribution	50.00%
Upper limit of employer match	4.00%
Maximum statutory compensation under code	$ 270,000
Percentage contributions under the savings plan, vested	100.00%
Contributions and plan administration costs for savings plan	$ 4,600,000	$ 4,100,000	$ 4,700,000