Changes in Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ 84,993		$ 99,547
Gains recognized in accumulated OCI, net of tax	17,733		38,561
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(25,551)		(60,933)
Other comprehensive loss	(7,818)	(97,959)	(22,372)	(105,405)
Balance at the end of the period	77,175		77,175
Cash flow hedge, net of tax | Natural gas
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	122,693		138,188
Gains recognized in accumulated OCI, net of tax	17,733		38,561
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	(26,020)		(62,343)
Other comprehensive loss	(8,287)		(23,782)
Balance at the end of the period	114,406		114,406
Cash flow hedge, net of tax | Interest rate
Changes in accumulated other comprehensive income (loss)
Balance at the beginning of the period	(1,204)		(1,276)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	36		108
Other comprehensive loss	36		108
Balance at the end of the period	(1,168)		(1,168)
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	(36,496)		(37,365)
Amounts reclassified from accumulated OCI into realized (income) expense, net of tax	433		1,302
Other comprehensive loss	433		1,302
Balance at the end of the period	$ (36,063)		$ (36,063)