Debt - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 02, 2019
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 1,667,487
Annualized interest rate	2.04%	2.93%
Letters of credit outstanding	$ 150,000	$ 155,000		$ 150,000
Remaining amount available under credit line	310,000	292,900
Average balance outstanding during period	$ 288,684	$ 368,228	$ 323,429
Daily-weighted average interest rate (annualized)	2.62%	2.40%	1.30%
Maximum daily amount outstanding during period	$ 385,000	$ 525,990	$ 455,912
Short-term Debt
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 290,000	$ 302,100