Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Short-Term Borrowings Outstanding	$ 81,900,000		$ 42,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility	120,000,000		120,000,000
Short-Term Borrowings Outstanding	81,900,000		42,000,000
Letters of Credit Outstanding	1,100,000	$ 1,100,000	0
Available revolving credit facility	$ 37,000,000		$ 78,000,000