Fair Value Measurements (Summary Of Financial Assets And Liabilities Accounted For At Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets:
Investments in marketable securities	$ 261,414	$ 239,652
Derivatives	27,061	32,315
Total assets	288,475
Liabilities:
Coal derivative liabilities	421	1,737
Level 1 [Member]
Assets:
Investments in marketable securities	101,338
Derivatives	22,750
Total assets	124,088
Liabilities:
Coal derivative liabilities
Level 2 [Member]
Assets:
Investments in marketable securities	160,076
Derivatives
Total assets	160,076
Liabilities:
Coal derivative liabilities	204
Level 3 [Member]
Assets:
Investments in marketable securities
Derivatives	4,311
Total assets	4,311
Liabilities:
Coal derivative liabilities	$ 217