Employee Benefit Plans - Additional Information (Detail) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discretionary profit sharing	$ 0	$ 0	$ 0
Contribution plan, employer matching contribution percentage	50.00%
Defined contribution plans employee contribution percentage of eligible compensation matched by employer	6.00%
Shares allocated to the employee stock ownership plan	0.7	0.8
Contribution expense under employee stock ownership plan	$ 4,700,000	$ 5,000,000	4,600,000
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 0
Discretionary profit sharing		7,600,000	$ 100,000
Settlement loss		9,800,000
Benefit cost, net periodic benefit cost		$ 11,600,000