Debt (Details) - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Debt
Line of credit and notes payable	$ 0	$ 18,000	$ 38,000
Revolving Credit Facility | Base Rate
Debt
Credit facility, interest rate basis	base rate
Line of Credit
Debt
Credit facility, total available credit	$ 304,300,000
Revolving Credit Facility
Debt
Credit facility, total available credit	300,000,000
Credit facility, unused borrowing capacity	$ 4,300,000
Credit facility, interest rate low end of range (as a percent)	0.76%
Credit facility, interest rate high end of range (as a percent)	7.25%
Line of credit and notes payable	$ 0
Revolving Credit Facility | Minimum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.15%
Revolving Credit Facility | Maximum
Debt
Facility fees on the available commitment of the facility (as a percent)	0.30%
Revolving Credit Facility | LIBOR
Debt
LIBOR rate	0.20%
Credit facility, interest rate basis	LIBOR
Revolving Credit Facility | LIBOR | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.60%
Revolving Credit Facility | LIBOR | Maximum
Debt
Credit facility, interest rate spread (as a percent)	1.45%
Revolving Credit Facility | Base Rate | Minimum
Debt
Credit facility, interest rate spread (as a percent)	0.00%
Revolving Credit Facility | Base Rate | Maximum
Debt
Credit facility, interest rate spread (as a percent)	0.45%