Incentive Plans - Summary of LFI and Other Similar Deferred Compensation Arrangements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangements [Abstract]
Prepaid Compensation Asset, Beginning Balance	$ 49,650	$ 75,703
Prepaid Compensation Asset, Granted	77,580	52,121
Prepaid Compensation Asset, Forfeited	(992)	(2,942)
Prepaid Compensation Asset, Amortization	(67,575)	(73,340)
Prepaid Compensation Asset, Other	1,692	(1,892)
Prepaid Compensation Asset, Ending Balance	60,355	49,650
Compensation Liability, Beginning Balance	170,388	193,574
Compensation Liability, Granted	77,580	52,121
Compensation Liability, Settled	(96,025)	(77,457)
Compensation Liability, Forfeited	(1,819)	(5,380)
Compensation Liability, Increase (Decrease) in fair value of underlying investments	23,526	3,318
Compensation Liability, Adjustment for estimated forfeitures	6,634	4,671
Compensation Liability, Other	2,017	(459)
Compensation Liability, Ending Balance	$ 182,301	$ 170,388