Employee Compensation and Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Description of 401 K Plan
Employee Benefits Disclosure [Line Items]
Maximum percentage of pre-tax income employees are allowed to contribute towards 401(k) plan	75.00%
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount	$ 18,000	$ 18,000	$ 18,000
Maximum percentage of employer matching contributions towards 401(k) plan	5.00%
Employee Stock Ownership Plan (ESOP), Plan
Employee Benefits Disclosure [Line Items]
Maximum percentage of employee's eligible pay earned in the fiscal year contributed by the company in cash or common stock towards ESOP	10.00%
ESOP contributions vesting period	5 years
Employee Home Ownership Plan
Employee Benefits Disclosure [Line Items]
Fixed-rate mortgage loan term, minimum	5 years
Fixed-rate mortgage loan term, maximum	7 years
Amortization period	30 years
Maximum loan amount percentage lesser of purchase price or the appraised value	80.00%
Mortgage loans rate	2.00%
Deferred Compensation Plan
Employee Benefits Disclosure [Line Items]
Maximum percentage of base salary that may be deferred under the DC Plan	50.00%
Maximum percentage of eligible bonus payment that may be deferred under the DC Plan	100.00%
Maximum deferral term under the DC Plan	12 months
Deferrals under the DC Plan	$ 5,500,000	4,400,000	3,700,000
DC Plan investment gains	$ 4,700,000	$ 2,400,000	$ 100,000