Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Debt Instrument [Line Items]
Short-Term Borrowings Outstanding	$ 64,800,000	$ 82,800,000	$ 37,400,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, limit	120,000,000.0	120,000,000.0	120,000,000.0
Short-Term Borrowings Outstanding	64,800,000	82,800,000	37,400,000
Available revolving credit facility	$ 55,200,000	$ 37,200,000	$ 82,600,000