Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Short-Term Borrowings Outstanding	$ 38,300,000	$ 81,900,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, limit	120,000,000	120,000,000
Short-Term Borrowings Outstanding	38,300,000	81,900,000
Letters of Credit Outstanding		1,100,000
Available revolving credit facility	$ 81,700,000	$ 37,000,000