Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Debt
Credit facility, maximum borrowing capacity	$ 304,300,000
Primary revolving line of credit
Debt
Credit facility, maximum borrowing capacity	300,000,000
Primary revolving line of credit | Minimum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.15%
Primary revolving line of credit | Maximum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.30%
Primary revolving line of credit | LIBOR
Debt
LIBOR Rate at end of period (as a percent)	0.18%
Credit facility, interest rate basis	LIBOR
Primary revolving line of credit | LIBOR | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.60%
Primary revolving line of credit | LIBOR | Maximum
Debt
Credit facility, interest rate spread (as a percent)	1.45%
Primary revolving line of credit | Base rate
Debt
Credit facility, interest rate basis	base rate
Primary revolving line of credit | Base rate | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.00%
Primary revolving line of credit | Base rate | Maximum
Debt
Credit facility, interest rate spread (as a percent)	0.45%
Other revolving credit lines and long term debt
Debt
Total borrowing capacity	4,300,000
Credit facility, interest rate low end of range (as a percent)	0.82%
Credit facility, interest rate high end of range (as a percent)	7.25%
Total outstanding balances	$ 0	$ 18,000	$ 83,000