Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Debt
Credit facility, maximum borrowing capacity	$ 307.5
Primary revolving line of credit
Debt
Credit facility, maximum borrowing capacity	300.0
Primary revolving line of credit | Maximum
Debt
Credit facility, interest rate spread (as a percent)	0.45%
Facility fees on the available commitment of the facility (as a percent)	0.30%
Primary revolving line of credit | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.00%
Facility fees on the available commitment of the facility (as a percent)	0.15%
Primary revolving line of credit | LIBOR
Debt
Credit facility, interest rate basis	LIBOR
LIBOR Rate at end of period (as a percent)	0.19%
Primary revolving line of credit | LIBOR | Maximum
Debt
Credit facility, interest rate spread (as a percent)	1.45%
Primary revolving line of credit | LIBOR | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.60%
Primary revolving line of credit | Base rate
Debt
Credit facility, interest rate basis	Base rate
Other revolving credit lines and long term debt
Debt
Total Borrowing Capacity	8.7
Credit facility, interest rate low end of range (as a percent)	1.018%
Credit facility, interest rate high end of range (as a percent)	11.515%
Total outstanding balances	$ 1.2	$ 0.2	$ 4.8