Short-Term Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Short-term Debt [Line Items]
Balance at end of period	$ 225,180		$ 225,000
Annualized interest rate	0.36%		0.55%
Letters of credit outstanding	198,000		198,000
Remaining amount available under credit line	176,820		177,000
Average balance outstanding during period	250,401		206,148		196,317
Daily-weighted average interest rate	0.41%	[1]	0.39%	[1]	0.43%	[1]
Maximum daily amount outstanding during period	$ 385,000		$ 271,761		$ 249,950

[1] Annualized