Long-Term Debt and Credit Facilities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Revolving credit facility
Debt instrument disclosure
Maximum borrowing capacity	$ 500		$ 500
Outstanding borrowings	40.0		40.0		0
Outstanding letters of credit	2.9		2.9		2.9
Credit agreement, base rate			London Interbank Offered Rate
Credit agreement, base rate margin (as a percent)			1.00%
Approximate interest on outstanding borrowings (as a percent)	1.77%	1.46%	1.77%	1.46%
Credit Agreement fees			$ 0.9
Term of amortization			5 years
Prior credit facility
Debt instrument disclosure
Credit agreement, base rate			London Interbank Offered Rate
Credit agreement, base rate margin (as a percent)			1.25%