Short-Term Borrowing - Additional Information (Detail)	1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 28, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Short-term Debt [Line Items]
Short-term borrowings		$ 175,644,000	$ 175,644,000	$ 247,371,000
Line of Credit Facility, Aggregate Borrowing Capacity		$ 375,000,000.0	$ 375,000,000.0
Number Of Unsecured Bank Credit Facilities			3
Short-term Debt, Weighted Average Interest Rate, at Point in Time		1.28%	1.28%	2.62%
Ratio of Indebtedness to Net Capital		0.65	0.65
Long-term debt including current maturities		$ 523,000,000.0	$ 523,000,000.0	$ 486,600,000
Line of Credit Facility, Commitment Fee Percentage			0.175%
Line of Credit Facility, Interest Rate During Period			1.125%
Line of Credit Facility, Remaining Borrowing Capacity		196,900,000	$ 196,900,000
Letters of Credit Outstanding, Amount		4,800,000	4,800,000
Notional Amount of Nonderivative Instruments		60,000,000.0	100,000,000.0
Subsequent Event
Short-term Debt [Line Items]
Notional Amount of Nonderivative Instruments	$ 40,000,000.0
Fixed Swap Rate	0.17%
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Aggregate Borrowing Capacity		$ 375,000,000	$ 375,000,000
Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 0.75 percent
Committed Line of Credit Facility Three [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			Lender's base rate, plus 0.75 percent
Interest Rate Swap Rate, Low Range [Member]
Short-term Debt [Line Items]
Fixed Swap Rate		0.20%	0.2615%
Interest Rate Swap Rate, High Range [Member]
Short-term Debt [Line Items]
Fixed Swap Rate		0.205%	0.3875%
Term Note Due February Two Thousand Twenty [Member]
Short-term Debt [Line Items]
Long-term debt including current maturities				$ 30,000,000