Allowance for Doubtful Accounts - Impaired Notes Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impaired Notes Receivable:
Balance beginning of year	$ 5,685	$ 6,471	$ 10,208
Additions	1,169	0	395
Deductions	0	786	4,132
Balance end of year	6,854	5,685	6,471
Average Outstanding balance	6,270	6,078	8,340
Reserve for Impaired Notes Receivable:
Balance beginning of year	2,419	2,139	3,031
Additions	465	280	99
Deductions	0	0	991
Balance end of year	$ 2,884	$ 2,419	$ 2,139