Bank note payable (Details) (USD $)	3 Months Ended		3 Months Ended																								3 Months Ended								3 Months Ended				3 Months Ended		3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 2012 Credit Facility	Mar. 31, 2013 2012 Credit Facility Less than or equal to 25%	Mar. 31, 2013 2012 Credit Facility Greater than 25% to less than or equal to 35%	Mar. 31, 2013 2012 Credit Facility Greater than 35% to less than or equal to 45%	Mar. 31, 2013 2012 Credit Facility Greater than 45% to less than or equal to 55%	Mar. 31, 2013 2012 Credit Facility Greater than 55%	Mar. 31, 2013 2012 Credit Facility Minimum	Mar. 31, 2013 2012 Credit Facility Minimum Greater than 25% to less than or equal to 35%	Mar. 31, 2013 2012 Credit Facility Minimum Greater than 35% to less than or equal to 45%	Mar. 31, 2013 2012 Credit Facility Minimum Greater than 45% to less than or equal to 55%	Mar. 31, 2013 2012 Credit Facility Minimum Greater than 55%	Mar. 31, 2013 2012 Credit Facility Maximum	Mar. 31, 2013 2012 Credit Facility Maximum Less than or equal to 25%	Mar. 31, 2013 2012 Credit Facility Maximum Greater than 25% to less than or equal to 35%	Mar. 31, 2013 2012 Credit Facility Maximum Greater than 35% to less than or equal to 45%	Mar. 31, 2013 2012 Credit Facility Maximum Greater than 45% to less than or equal to 55%	Mar. 31, 2013 2012 Credit Facility LIBOR	Mar. 31, 2013 2012 Credit Facility LIBOR Less than or equal to 25%	Mar. 31, 2013 2012 Credit Facility LIBOR Greater than 25% to less than or equal to 35%	Mar. 31, 2013 2012 Credit Facility LIBOR Greater than 35% to less than or equal to 45%	Mar. 31, 2013 2012 Credit Facility LIBOR Greater than 45% to less than or equal to 55%	Mar. 31, 2013 2012 Credit Facility LIBOR Greater than 55%	Mar. 31, 2013 2012 Credit Facility LIBOR Minimum	Mar. 31, 2013 2012 Credit Facility LIBOR Maximum	Mar. 31, 2013 2012 Credit Facility Bank's base rate	Mar. 31, 2013 2012 Credit Facility Bank's base rate Less than or equal to 25%	Mar. 31, 2013 2012 Credit Facility Bank's base rate Greater than 25% to less than or equal to 35%	Mar. 31, 2013 2012 Credit Facility Bank's base rate Greater than 35% to less than or equal to 45%	Mar. 31, 2013 2012 Credit Facility Bank's base rate Greater than 45% to less than or equal to 55%	Mar. 31, 2013 2012 Credit Facility Bank's base rate Greater than 55%	Mar. 31, 2013 2012 Credit Facility Bank's base rate Minimum	Mar. 31, 2013 2012 Credit Facility Bank's base rate Maximum	Mar. 31, 2013 2012 Credit Facility Federal funds rate	Mar. 31, 2013 2012 Credit Facility One month LIBOR	Mar. 31, 2013 2012 Revolver	Sep. 27, 2012 2012 Revolver	Mar. 31, 2013 2012 Term Loan	Sep. 27, 2012 2012 Term Loan	Mar. 31, 2013 2012 Term Loan LIBOR	Mar. 31, 2013 2011 Revolver	Dec. 31, 2012 2011 Revolver	Sep. 27, 2012 2011 Revolver
Bank note and term note payable
Total available			$ 900,000,000																																		$ 500,000,000		$ 400,000,000			$ 600,000,000
Borrowings outstanding	221,750,000	216,750,000																																			221,750,000	82,000,000		400,000,000			216,750,000	482,000,000
Term of the borrowing																																					4 years		5 years
Additional borrowing capacity allowed by exercising an accordion feature																																					$ 250,000,000
Extension available on debt																																					12 months
Variable base rate																			LIBOR								Bank's base rate								Federal Funds Rate	One month LIBOR					LIBOR
Basis spread on variable rate (as a percent)																			1.45%	1.35%	1.40%	1.45%	1.65%	1.90%	1.35%	1.90%	0.45%	0.35%	0.40%	0.45%	0.65%	0.90%	0.35%	0.90%	0.50%	1.00%
Fixed rate (as a percent)																																									0.75%
Overall rate (basis spread plus variable rate), end of period (as a percent)																			1.65%								3.70%												2.20%				2.23%
Weighted average rate interest rate, during period (as a percent)	1.66%																																				1.65%
Leverage Ratio										25.00%	35.00%	45.00%	55.00%		25.00%	35.00%	45.00%	55.00%
Facility fee (as a percent)			0.30%	0.20%	0.25%	0.30%	0.35%	0.40%	0.20%					0.40%