Stock-based Compensation Plans - Additional Information 4 (Detail) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan vesting period			3 years
Deferred compensation plan	$ (223,000)	$ 9,203,000	$ 559,000	$ 36,838,000
Shares held in rabbi trust total	2.9		2.9		2.9
Vested shares held in rabbi trust	2.7		2.7		2.8
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Estimated prior service cost amortized from accumulated other comprehensive income into general and administrative expense			$ 276,000