Incentive Plans - Summary of LFI and Other Similar Deferred Compensation Arrangements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement With Individual Excluding Share Based Payments And Postretirement Benefits [Line Items]
Prepaid Compensation Asset, Beginning Balance	$ 60,355
Prepaid Compensation Asset, Ending Balance	76,362	$ 60,355
Compensation Liability, Beginning Balance	198,417
Compensation Liability, Ending Balance	188,962	198,417
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	104,740	77,580
Prepaid Compensation Asset, Forfeited	(1,391)	(992)
Prepaid Compensation Asset, Amortization	(85,679)	(67,575)
Prepaid Compensation Asset, Other	(1,663)	1,692
Prepaid Compensation Asset, Ending Balance	76,362	60,355
Compensation Liability, Beginning Balance	182,301	170,388
Compensation Liability, Granted	104,740	77,580
Compensation Liability, Settled	(86,318)	(96,025)
Compensation Liability, Forfeited	(2,682)	(1,819)
Compensation Liability, Increase (decrease) in fair value of underlying investments	(14,086)	23,526
Compensation Liability, Adjustment for estimated forfeitures	5,842	6,634
Compensation Liability, Other	(1,775)	2,017
Compensation Liability, Ending Balance	$ 188,022	$ 182,301