Debt - Short-Term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 1,416,109
Annualized interest rate	0.96%	0.55%
Letters of credit outstanding	$ 177,000	$ 177,000
Remaining amount available under credit line	310,150	332,456
Average balance outstanding during period	$ 301,550	$ 350,851	$ 296,246
Daily-weighted average interest rate (annualized)	0.83%	0.43%	0.26%
Maximum daily amount outstanding during period	$ 412,676	$ 458,110	$ 343,000
Short-term Debt
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 262,850	$ 240,544