Debt - Short-Term Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Disclosure [Abstract]
Balance at end of period (at par value)	$ 229,140	$ 225,180
Annualized interest rate	0.30%	0.36%
Letters of credit outstanding	198,000	198,000
Remaining amount available under credit line	172,860	176,820
Average balance outstanding during period	274,435	250,401	206,148
Daily-weighted average interest rate	0.31%	0.41%	0.39%
Maximum daily amount outstanding during period	$ 340,140	$ 385,000	$ 271,761