Debt - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 14, 2018
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 1,686,462
Annualized interest rate	2.93%	1.78%
Letters of credit outstanding	$ 155,000	$ 177,000		$ 155,000
Remaining amount available under credit line	292,900	248,750
Average balance outstanding during period	$ 368,228	$ 323,429	$ 301,550
Daily-weighted average interest rate (annualized)	2.40%	1.30%	0.83%
Maximum daily amount outstanding during period	$ 525,990	$ 455,912	$ 412,676
Short-term Debt
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 302,100	$ 324,250