Comparison of credit facilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Credit Facilities	$ 1,200
Term Loan
Line of Credit Facility [Line Items]
Credit Facilities	300	650
Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit Facilities	900	300
Funded letter of credit facility
Line of Credit Facility [Line Items]
Credit Facilities		320
Letters of credit
Line of Credit Facility [Line Items]
Credit Facilities	$ 900	$ 520