Incentive Plans - Summary of LFI and Other Similar Deferred Compensation Arrangements (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Deferred Compensation Arrangement With Individual Excluding Share Based Payments And Postretirement Benefits [Line Items]
Compensation Liability, Beginning Balance	$ 198,417
Compensation Liability, Ending Balance	202,975
LFI and Other Similar Deferred Compensation Arrangements [Member]
Deferred Compensation Arrangement With Individual Excluding Share Based Payments And Postretirement Benefits [Line Items]
Prepaid Compensation Asset, Beginning Balance	60,355	$ 49,650
Prepaid Compensation Asset, Granted	104,551	77,580
Prepaid Compensation Asset, Forfeited	(618)	(585)
Prepaid Compensation Asset, Amortization	(50,057)	(39,503)
Prepaid Compensation Asset, Other	(1,424)	942
Prepaid Compensation Asset, Ending Balance	112,807	88,084
Compensation Liability, Beginning Balance	182,301	170,388
Compensation Liability, Granted	104,551	77,580
Compensation Liability, Settled	(84,422)	(95,633)
Compensation Liability, Forfeited	(876)	(933)
Compensation Liability, (Decrease) Increase in fair value of underlying investments	(1,935)	13,106
Compensation Liability, Adjustment for estimated forfeitures	2,330	2,138
Compensation Liability, Other	(1,105)	1,312
Compensation Liability, Ending Balance	$ 200,844	$ 167,958