Long-Term Debt and Financing Arrangements - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Revolving Line of Credit Facility [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 350,000,000
Borrowings under bank revolving line of credit		$ 45,000,000
Revolving Line of Credit Facility [Member] | Bank Revolving Line of Credit [Member]
Line Of Credit Facility [Line Items]
Unused and available borrowings under bank revolving line of credit	327,000,000	284,900,000
Borrowings under bank revolving line of credit	$ 0	$ 45,000,000
Standby Letters of Credit [Member]
Line Of Credit Facility [Line Items]
Letters of credit expiration year one	2018
Letters of credit expiration year two	2019
Outstanding letters of credit	$ 23,000,000