Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Short-term debt	$ 185,000	$ 22,000	$ 124,000
Long-term debt
Total current maturities of long-term debt	13,700	54,205	62,482
Long-term debt, less current maturities	435,580	418,339	354,066
Nonrecourse
Debt Instrument [Line Items]
Short-term debt	0	0	0
Long-term debt
Total current maturities of long-term debt	2,922	0	0
Long-term debt, less current maturities	72,290	0	0
Recourse
Debt Instrument [Line Items]
Short-term debt	185,000	22,000	124,000
Long-term debt
Total current maturities of long-term debt	10,778	54,205	62,482
Long-term debt, less current maturities	$ 363,290	$ 418,339	$ 354,066