Postretirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions	$ 52,800,000	$ 21,300,000	$ 15,800,000
Maximum allowed investment percentage in any single issuer in pension plan assets at time of purchase	10.00%
Accumulated benefit obligation	$ 505,898,000	540,963,000
Maximum
Defined Benefit Plan Disclosure [Line Items]
Capped eligible compensation	1,000,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions	$ 52,781,000	$ 21,272,000