Debt - Short-Term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 1,243,803,000
Annualized interest rate	0.55%	0.32%
Letters of credit outstanding	$ 177,000,000	$ 198,000,000
Remaining amount available under credit line	332,456,000	313,550,000
Average balance outstanding during period	$ 350,851,000	$ 296,246,000	$ 274,435,000
Daily-weighted average interest rate (annualized)	0.43%	0.26%	0.33%
Maximum daily amount outstanding during period	$ 458,110,000	$ 343,000,000	$ 340,140,000
Short-term Debt [Member]
Short-term Debt [Line Items]
Balance at end of period (at par value)	$ 240,544,000	$ 238,450,000