Debt (Recourse Debt) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Long-term debt
Less: current maturities		$ 21,589	$ 54,205
Long-term debt, less current maturities		496,187	418,339
Collateralized debt book value		143,800
Recourse
Long-term debt
Long-term debt, gross		371,791	476,220
Less: current maturities		16,747	54,205
Less: unamortized prepaid debt issuance costs		5,210	3,676
Long-term debt, less current maturities		349,834	418,339
Recourse | Note payable, 4.07%, payable at maturity, due 2021
Long-term debt
Long-term debt, gross		$ 26,000	26,000
Interest rate of debt instruments		4.07%
Recourse | Note payable, 4.55%, payable at maturity, due 2023
Long-term debt
Long-term debt, gross		$ 24,000	24,000
Interest rate of debt instruments		4.55%
Recourse | Note payable, 4.85%, payable at maturity, due 2026
Long-term debt
Long-term debt, gross		$ 25,000	25,000
Interest rate of debt instruments		4.85%
Recourse | Note payable, 6.78%, payable at maturity, paid 2018
Long-term debt
Long-term debt, gross		$ 0	41,500
Interest rate of debt instruments		6.78%
Recourse | Note payable, 4.92%, payable in increasing amounts ($2.0 million for 2018), plus interest, due 2021 (a)
Long-term debt
Long-term debt, gross	[1]	$ 16,227	18,241
Interest rate of debt instruments		4.92%
Annual payments		$ 2,000
Recourse | Note payable, 4.76%, payable in increasing amounts ($1.6 million for 2018) plus interest, due 2028 (a)
Long-term debt
Long-term debt, gross	[1]	$ 44,330	45,936
Interest rate of debt instruments		4.76%
Annual payments		$ 1,600
Recourse | Note payable, variable rate (4.85% at December 31, 2018), payable in increasing amounts ($1.3 million for 2018) plus interest, due 2023 (a)
Long-term debt
Long-term debt, gross	[1]	$ 16,452	17,786
Interest rate of debt instruments		4.85%
Annual payments		$ 1,300
Recourse | Note payable, 3.29%, payable in increasing amounts ($1.4 million for 2018) plus interest, due 2022 (a)
Long-term debt
Long-term debt, gross	[1]	$ 18,918	20,293
Interest rate of debt instruments		3.29%
Annual payments		$ 1,400
Recourse | Note payable, 4.23%, payable quarterly in varying amounts ($0.5 million for 2018) plus interest, due 2021 (a)
Long-term debt
Long-term debt, gross	[1]	$ 9,948	10,479
Interest rate of debt instruments		4.23%
Annual payments		$ 500
Recourse | Note payable, variable rate (4.19% at December 31, 2018), payable in varying amounts ($0.3 million for 2018), plus interest, due 2026 (a)
Long-term debt
Long-term debt, gross	[1]	$ 8,417	8,762
Interest rate of debt instruments		4.19%
Annual payments		$ 300
Recourse | Note payable, 4.76%, payable quarterly in varying amounts ($0.3 million for 2018) plus interest, due 2028 (a)
Long-term debt
Long-term debt, gross	[1]	$ 8,288	8,581
Interest rate of debt instruments		4.76%
Annual payments		$ 300
Recourse | Line of credit, variable rate (3.79% at December 31, 2018), payable at maturity, due 2022
Long-term debt
Long-term debt, gross		$ 50,000	100,000
Interest rate of debt instruments		3.79%
Annual payments		$ 50,000
Recourse | Note payable, 3.33%, payable in increasing amounts ($1.1 million for 2018) plus interest, due 2025 (a)
Long-term debt
Long-term debt, gross	[1]	$ 24,888	25,960
Interest rate of debt instruments		3.33%
Annual payments		$ 1,100
Recourse | Note payable, 4.5%, payable at maturity, due 2030
Long-term debt
Long-term debt, gross		$ 16,000	16,000
Interest rate of debt instruments		4.50%
Recourse | Note payable, 5.0%, payable at maturity, due 2040
Long-term debt
Long-term debt, gross		$ 14,000	14,000
Interest rate of debt instruments		5.00%
Recourse | Note payable, variable rate (4.15% at December 31, 2018), payable quarterly, ($1.25 million for 2018) due 2024 (a)
Long-term debt
Long-term debt, gross		$ 13,250	14,500
Interest rate of debt instruments		4.15%
Annual payments		$ 1,300
Recourse | Variable rate (3.19% at December 31, 2018), payable at maturity, due 2019 (a)
Long-term debt
Long-term debt, gross	[1]	$ 4,650	4,650
Interest rate of debt instruments		3.19%
Recourse | Variable rate (3.18% at December 31, 2018), payable at maturity, due 2025 (a)
Long-term debt
Long-term debt, gross	[1]	$ 3,100	3,100
Interest rate of debt instruments		3.18%
Recourse | Variable rate (3.14% at December 31, 2018), payable at maturity, due 2036
Long-term debt
Long-term debt, gross		$ 21,000	21,000
Interest rate of debt instruments		3.14%
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032
Long-term debt
Long-term debt, gross		$ 27,323	$ 30,432
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032 | Minimum
Long-term debt
Interest rate of debt instruments		2.65%
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032 | Maximum
Long-term debt
Interest rate of debt instruments		5.00%

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $143.8 million