Debt - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 1,475,634
Annualized interest rate	1.78%	0.96%
Letters of credit outstanding	$ 177,000	$ 177,000
Remaining amount available under credit line	248,750	310,150
Average balance outstanding during period	$ 323,429	$ 301,550	$ 350,851
Daily-weighted average interest rate (annualized)	1.30%	0.83%	0.43%
Maximum daily amount outstanding during period	$ 455,912	$ 412,676	$ 458,110
Short-term Debt
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 324,250	$ 262,850