Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Guarantee	Sep. 30, 2012 $1 billion revolver weighted average rate 1.71%, due in 2016	Dec. 31, 2011 $1 billion revolver weighted average rate 1.71%, due in 2016	Sep. 30, 2012 $100 million Private Placement notes 5.64%, due in 2015	Dec. 31, 2011 $100 million Private Placement notes 5.64%, due in 2015	Sep. 30, 2012 $175 million Private Placement notes 3.89%, due in 2017	Dec. 31, 2011 $175 million Private Placement notes 3.89%, due in 2017	Sep. 30, 2012 $225 million Private Placement notes 4.47%, due in 2020	Dec. 31, 2011 $225 million Private Placement notes 4.47%, due in 2020
Debt Instrument [Line Items]
Maximum borrowing capacity of line of credit facility			$ 1,000	$ 1,000
Debt maturity period	2012-12		2016-09		2015-04		2017-10		2020-10
Long-term debt, face amount					100.0	100.0	175.0	175.0	225.0	225.0
Senior letters of credit facility, committed to outstanding letters of credit			158.4	159.1
Senior letters of credit facility, unused portion			609.8	313.0
Outstanding short term loan	200.0
Bank loan guaranteed on behalf of Shiraishi	6.3
Debt maturity period		May 31, 2013
Notes receivable extended to Shiraishi	$ 15.4