Debt (Details Textual) (USD $)	1 Months Ended	12 Months Ended				12 Months Ended					12 Months Ended
	Sep. 30, 2009	Dec. 31, 2011 Institution	Dec. 31, 2009	Jun. 30, 2011	Dec. 31, 2010	Dec. 31, 2011 Medium Term Notes Maturing in May 2011 [Member]	Dec. 31, 2011 Medium Term Notes Maturing In April 2010 [Member]	Jun. 30, 2011 Letter of Credit [Member]	Jun. 30, 2011 Global Revolving Credit Facility [Member]	Dec. 31, 2011 Credit Facility Due on April 2012 [Member]	Dec. 31, 2011 Medium-term Notes [Member]	May 31, 2011 350 million unsecured medium-term notes due June 2017 [Member]	May 31, 2011 350 million unsecured medium-term notes due June 2017 [Member] Interest rate swaps maturing June 2017 [Member]	Feb. 28, 2011 350 million unsecured medium-term notes due March 2015 [Member]	Feb. 28, 2011 350 million unsecured medium-term notes due March 2015 [Member] Interest rate swaps maturing March 2015 [Member]
Debt (Textual) [Abstract]
Commercial paper classified as long term debt		$ 415,936,000			$ 367,880,000
Repurchase price condition of notes											101% of Principal Plus Accrued And Unpaid Interest
Face amount of unsecured medium-term notes issued												350,000,000		350,000,000
Aggregate notional amount of interest rate swaps		550,000,000			250,000,000								150,000,000		150,000,000
Line of Credit Facility [Line Items]
Maximum borrowing capacity under global revolving credit facility								75,000,000	900,000,000	875,000,000
Short-term Debt [Line Items]
Debt Tender Offer	100,000,000		100,000,000			50,000,000	50,000,000
Percentage of debt retired under debt tender offer						5.95%	4.625%
Unamortized original issue discounts		9,000,000			10,000,000
Number of lending institutions		12
Letter of credit outstanding amount				0
Annual facility fees minimum		0.10%
Annual facility fees maximum		0.325%
Current annual facility fee		0.15%
Total facility size		900,000,000
Global revolving credit facility covenant terms, debt to consolidated tangible net worth ratio		less than or equal to 300%
Tangible net worth definition		Includes 50% of Deferred Federal Income Tax Liability and Excludes Intangibles
Debt to consolidated tangible net worth ratio		255.00%
Amount available under the global revolving credit facility		483,000,000
Total available proceeds under trade receivables purchase and sale program		175,000,000
Amount outstanding under trade receivables purchase and sale program		0			0
Number of days under trade trade receivables purchase and sale program		364 days
Debt retirement cost	104,000,000
Pre-tax extinguishment charge		4,000,000
Pre-tax debt extinguishment premium paid	3,000,000		3,000,000
Write-off of unamortized original debt discount and issuance cost		$ 1,000,000