Pension and Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure	$ 35,700,000	$ 33,300,000
Available-for-sale Securities	$ 2,537,200,000	2,396,400,000
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	$ 300,000
Postretirement plan's estimated future subsidies	300,000
Employer contribution	$ 13,000,000.0
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	$ 6,300,000	5,100,000	$ 5,100,000
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)	2.00%
Postretirement Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 0	0
Postretirement Benefit Costs | Gross Benefit Payments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments	1,400,000
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	200,900,000	177,600,000
Postretirement plan's gross benefit payments	11,300,000
Pension | International funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure	$ 31,300,000	$ 18,600,000