Incentive Plans - Summary of LFI and Other Similar Deferred Compensation Arrangements (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Deferred Compensation Arrangements [Abstract]
Prepaid Compensation Asset, Beginning Balance	$ 60,355	$ 49,650
Prepaid Compensation Asset, Granted	103,947	77,580
Prepaid Compensation Asset, Forfeited	(84)	(93)
Prepaid Compensation Asset, Amortization	(15,373)	(12,491)
Prepaid Compensation Asset, Other	251	(12)
Prepaid Compensation Asset, Ending Balance	149,096	114,634
Compensation Liability, Beginning Balance	182,301	170,388
Compensation Liability, Granted	103,947	77,580
Compensation Liability, Settled	(72,979)	(81,559)
Compensation Liability, Forfeited	(138)	(133)
Compensation Liability, (Decrease) Increase in fair value of underlying investments	(1,436)	7,353
Compensation Liability, Adjustment for estimated forfeitures	517	532
Compensation Liability, Other	204	579
Compensation Liability, Ending Balance	$ 212,416	$ 174,740