Changes in Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 21,582		$ 99,547
Gains recognized in accumulated OCI, net of tax	73,429		20,828
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(10,018)		(35,382)
Other comprehensive income (loss)	63,411	(70,333)	(14,554)	(7,446)
Balance at the end of the period	84,993		84,993
Cash flow hedge, net of tax | Natural gas
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	59,754		138,188
Gains recognized in accumulated OCI, net of tax	73,429		20,828
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(10,490)		(36,323)
Other comprehensive income (loss)	62,939		(15,495)
Balance at the end of the period	122,693		122,693
Cash flow hedge, net of tax | Interest rate
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	(1,240)		(1,276)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	36		72
Other comprehensive income (loss)	36		72
Balance at the end of the period	(1,204)		(1,204)
Pension and other post-retirement benefits liability adjustment, net of tax
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	(36,932)		(37,365)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	436		869
Other comprehensive income (loss)	436		869
Balance at the end of the period	$ (36,496)		$ (36,496)