Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Short-Term Borrowings Outstanding	$ 45,300,000	$ 38,300,000	$ 76,600,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, limit	120,000,000	120,000,000	120,000,000
Short-Term Borrowings Outstanding	45,300,000	38,300,000	76,600,000
Letters of Credit Outstanding	0	0	1,100,000
Available revolving credit facility	$ 74,700,000	$ 81,700,000	$ 42,300,000