12. Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2015 |
Sep. 30, 2015 |
Jun. 30, 2015 |
Mar. 31, 2015 |
Dec. 31, 2014 |
Sep. 30, 2014 |
Jun. 30, 2014 |
Mar. 31, 2014 |
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Quarterly Financial Information Disclosure [Abstract] | |||||||||||
| Operating Revenue | $ 32,372 | $ 37,381 | $ 37,655 | $ 38,324 | $ 32,693 | $ 36,625 | $ 35,025 | $ 36,419 | $ 145,733 | $ 140,762 | $ 131,993 |
| Operating Income | 8,330 | 11,573 | 11,120 | 11,486 | 8,632 | 11,226 | 10,257 | 10,700 | 42,510 | 40,817 | 37,944 |
| Net Income | $ 6,050 | $ 7,799 | $ 7,474 | $ 7,602 | $ 6,040 | $ 7,685 | $ 6,882 | $ 7,201 | $ 28,925 | $ 27,808 | $ 26,582 |
| Income Per Basic Share | $ 3.32 | $ 4.25 | $ 4.04 | $ 4.05 | $ 3.11 | $ 3.94 | $ 3.51 | $ 3.63 | $ 15.67 | $ 14.20 | $ 13.22 |
| Income Per Diluted Share | $ 3.27 | $ 4.19 | $ 3.99 | $ 4.01 | $ 3.08 | $ 3.91 | $ 3.48 | $ 3.61 | $ 15.47 | $ 14.08 | $ 13.18 |