Property and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Components of property and equipment
Total, at cost	$ 902,930	$ 844,652
Less accumulated depreciation	(261,133)	(249,528)
Property and equipment, net	641,797	595,124
Land [Member]
Components of property and equipment
Total, at cost	160,490	142,730
Building and improvements [Member]
Components of property and equipment
Total, at cost	506,734	476,846
Office equipment and fixtures [Member]
Components of property and equipment
Total, at cost	118,658	115,509
Parts and service equipment [Member]
Components of property and equipment
Total, at cost	65,589	62,678
Company vehicles [Member]
Components of property and equipment
Total, at cost	7,916	7,750
Construction in progress [Member]
Components of property and equipment
Total, at cost	$ 43,543	$ 39,139