Financing Arrangements (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Apr. 30, 2011
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 200,000,000		$ 200,000,000		$ 125,000,000
Revolving credit facility, amount outstanding	0	0	0	0
Expiration date of revolving credit facility			June 2013
Annual commitment fee on undrawn amounts of revolving credit facility			0.375%
Line of Credit Facility, Commitment Fee Percentage, Rate Spread Over Bank Base Rate			0.50%
Line of Credit Facility, Commitment Fee Percentage, Rate Spread Over LIBOR			1.50%
Line of Credit Facility, Commitment Fee Amount	$ 200,000	$ 100,000	$ 500,000	$ 300,000