Schedule of Fair Value and Carrying Value of Assets and Liabilities Recorded at Other Than Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value | Level 2
Assets
Assets, fair value disclosure	$ 68,677	$ 68,629
Liabilities
Long-term debt, including current maturities	517,500	524,350
Total liabilities	517,500	524,350
Carrying Value
Assets
Assets, fair value disclosure	68,713	68,665
Liabilities
Long-term debt, including current maturities	500,000	500,000
Total liabilities	500,000	500,000
Money Market Funds | Fair Value | Level 2
Assets
Assets, fair value disclosure	31	41
Money Market Funds | Carrying Value
Assets
Assets, fair value disclosure	31	41
Commercial Paper | Fair Value | Level 2
Assets
Assets, fair value disclosure	63,026	62,440
Commercial Paper | Carrying Value
Assets
Assets, fair value disclosure	63,061	62,476
Corporate Bonds | Fair Value | Level 2
Assets
Assets, fair value disclosure	5,620	6,148
Corporate Bonds | Carrying Value
Assets
Assets, fair value disclosure	$ 5,621	$ 6,148