Allowance for Doubtful Accounts - Impaired Notes Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 43,043	$ 15,476
Impaired Notes Receivable:
Balance beginning of year	6,854	5,685	$ 6,471
Additions	23,382	1,169	0
Deductions	4,532	0	786
Balance end of year	25,704	6,854	5,685
Average Outstanding balance	15,448	6,270	6,078
Reserve for Impaired Notes Receivable:
Balance beginning of year	2,884	2,419	2,139
Additions	12,526	465	280
Deductions	1,938	0	0
Balance end of year	13,472	$ 2,884	$ 2,419
Note Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 10,000