Borrowing Limits Amounts Outstanding and Amounts Available under Credit Facility (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Short-Term Borrowings Outstanding	$ 69,100,000	$ 38,300,000	$ 111,900,000
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, limit	120,000,000	120,000,000	120,000,000
Short-Term Borrowings Outstanding	69,100,000	38,300,000	111,900,000
Letters of Credit Outstanding			1,100,000
Available credit facility	$ 50,900,000	$ 81,700,000	$ 7,000,000