Changes in Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 44,703	$ 99,547
Losses recognized in accumulated OCI, net of tax	(39,194)	(52,601)
Amounts reclassified from accumulated OCI into realized expense, net of tax	17,475	(25,364)
Other comprehensive (loss) income	(21,719)	(77,965)
Balance at the end of the period	22,984	21,582
Cash flow hedge, net of tax | Natural gas
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	61,699	138,188
Losses recognized in accumulated OCI, net of tax	(39,194)	(52,601)
Amounts reclassified from accumulated OCI into realized expense, net of tax	17,263	(25,833)
Other comprehensive (loss) income	(21,931)	(78,434)
Balance at the end of the period	39,768	59,754
Cash flow hedge, net of tax | Interest rate
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	(1,132)	(1,276)
Amounts reclassified from accumulated OCI into realized expense, net of tax	36	36
Other comprehensive (loss) income	36	36
Balance at the end of the period	(1,096)	(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	(15,864)	(37,365)
Amounts reclassified from accumulated OCI into realized expense, net of tax	176	433
Other comprehensive (loss) income	176	433
Balance at the end of the period	$ (15,688)	$ (36,932)