Pension and Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Document Period End Date		Dec. 31, 2017
Other Invested Assets Fair Value Disclosure		$ 45,200,000	$ 35,700,000
Available-for-sale Securities		$ 2,614,100,000	2,537,200,000
Eligible age of years to participate in benefit plans		20 years
Eligible years of credit service required to participate in benefit plans		5 years
Maximum age of years required to participate in benefit plans		65 years
Postretirement plan's subsidy related to Medicare Prescription Drug Improvement and Modernization Act of 2003	$ 200,000
Postretirement plan's estimated future subsidies		$ 100,000
Employer contribution		$ 10,000,000.0
Contributions of participant salary (percent)		1.00%
Matching contribution on participant's salary		$ 1
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		6.00%
Matching contribution by the employer		$ 0.50
Company's share of expense under the plan		$ 6,400,000	6,300,000	$ 5,100,000
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)		2.00%
Private Equity Funds, Foreign [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure			11,300,000
Postretirement Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments		$ 1,800,000
Employer contribution		0	0
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments		11,200,000
Pension | Private Equity Funds, Foreign [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure		$ 26,600,000	$ 20,000,000.0
Other Invested Assets Fair Value Disclosure		2