Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Short-term debt	$ 132,000	$ 22,000	$ 19,000
Long-term debt
Total current maturities of long-term debt	15,677	54,205	53,972
Long-term debt, less current maturities	437,280	418,339	371,315
Nonrecourse
Debt Instrument [Line Items]
Short-term debt	0	0	0
Long-term debt
Total current maturities of long-term debt	3,772	0	0
Long-term debt, less current maturities	77,114	0	0
Recourse
Debt Instrument [Line Items]
Short-term debt	132,000	22,000	19,000
Long-term debt
Total current maturities of long-term debt	11,905	54,205	53,972
Long-term debt, less current maturities	$ 360,166	$ 418,339	$ 371,315