Notes and Contracts Receivable - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 5,378	$ 6,470	$ 7,110
Provision for losses	4,181	3,926	4,055
Write-offs	(4,014)	(5,018)	(4,695)
Ending balance	$ 5,545	$ 5,378	$ 6,470