Debt - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 25, 2020	Nov. 24, 2020
Short-term Debt [Line Items]
Annualized interest rate	0.27%	2.04%
Letters of credit outstanding	$ 135,000	$ 150,000		$ 135,000	$ 150,000
Remaining amount available under credit line	360,000	310,000
Average balance outstanding during period	$ 318,409	$ 288,684	$ 368,228
Daily-weighted average interest rate (annualized)	1.50%	2.62%	2.40%
Maximum daily amount outstanding during period	$ 482,000	$ 385,000	$ 525,990
Short-term Debt
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 255,000	$ 290,000