Credit Facilities (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Mar. 07, 2012 Revolving credit facility
Debt instrument disclosure
Maximum borrowing capacity								$ 500
Outstanding borrowings						325.0	320.0
Outstanding letters of credit						2.9	2.9
Credit agreement, base rate	London Interbank Offered Rate			London Interbank Offered Rate
Credit agreement, base rate margin (as a percent)	2.00%	1.25%		1.25%
Approximate interest on outstanding borrowings (as a percent)		1.46%	1.50%	1.46%	1.68%
Credit Agreement fees	$ 1.9
Term of amortization	5 years
Capital stock of foreign subsidiaries pledged (as a percent)				65.00%