Debt (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Debt Instrument [Line Items]
Borrowings under short-term line of credit	$ 50,000	$ 24,219	$ 309,608
Long-term debt
Total current maturities of long-term debt	45,096	15,145	29,647
Total long-term debt, less current maturities	409,020	427,243	317,648
Nonrecourse
Debt Instrument [Line Items]
Borrowings under short-term line of credit	0	4,219	1,608
Long-term debt
Total current maturities of long-term debt	3,274	2,496	1,385
Total long-term debt, less current maturities	19,621	20,067	32,544
Recourse
Debt Instrument [Line Items]
Borrowings under short-term line of credit	50,000	20,000	308,000
Long-term debt
Total current maturities of long-term debt	41,822	12,649	28,262
Total long-term debt, less current maturities	$ 389,399	$ 407,176	$ 285,104