Fair Value Measurements (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	$ 299,941	$ 209,597
Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	317,440	218,550
Level 2 | Notes payable | Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	263,745	179,160
Level 2 | Notes payable | Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	279,505	186,918
Level 2 | Nonrecourse | Notes payable | Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	785	954
Level 2 | Nonrecourse | Notes payable | Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	800	966
Level 1 | Debenture bonds | Carrying Amount
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	35,411	29,483
Level 1 | Debenture bonds | Fair Value
Fair value of long-term debt estimated using quoted market prices or discounted future cash flows
Fair value of long-term debt	$ 37,135	$ 30,666