Changes in Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 99,547
Losses recognized in accumulated OCI, net of tax	(52,601)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(25,364)
Other comprehensive (loss) income	(77,965)	62,887
Balance at the end of the period	21,582
Cash flow hedge, net of tax | Natural gas
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	138,188
Losses recognized in accumulated OCI, net of tax	(52,601)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(25,833)
Other comprehensive (loss) income	(78,434)
Balance at the end of the period	59,754
Cash flow hedge, net of tax | Interest rate
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	(1,276)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	36
Other comprehensive (loss) income	36
Balance at the end of the period	(1,240)
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	(37,365)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	433
Other comprehensive (loss) income	433
Balance at the end of the period	$ (36,932)