Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Short-term debt	$ 489,000	$ 22,000	$ 255,000
Long-term debt
Total current maturities of long-term debt	14,134	54,205	56,144
Long-term debt, less current maturities	438,628	418,339	365,971
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	72,977	0	0
Recourse
Debt Instrument [Line Items]
Short-term debt	489,000	22,000	255,000
Long-term debt
Total current maturities of long-term debt	11,212	54,205	56,144
Long-term debt, less current maturities	$ 365,651	$ 418,339	$ 365,971