|
Inventories (Details) (USD $)
|
Mar. 31, 2015
|
Dec. 31, 2014
|
|---|---|---|
| Inventory Disclosure [Abstract] | ||
| Coal | $ 117,180,000 | $ 71,901,000 |
| Repair parts and supplies | 122,933,000 | 118,352,000 |
| Inventories | 240,113,000 | 190,253,000 |
| Allowance for slow-moving and obsolete inventories | $ 6,800,000 | $ 6,600,000 |