Changes in Accumulated Other Comprehensive Loss (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	$ (74,902)	$ (107,931)	$ (78,053)	$ (114,008)
OCI (loss) before reclassifications	(2,032)	(1,795)	(2,453)	(1,737)
Amounts reclassified from accumulated other comprehensive loss	2,717	5,269	6,289	11,288
Total other comprehensive income	685	3,474	3,836	9,551
Ending balance	(74,217)	(104,457)	(74,217)	(104,457)
Derivative Instruments
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(20,364)	(44,481)	(23,058)	(49,592)
OCI (loss) before reclassifications	40	(160)	(381)	(102)
Amounts reclassified from accumulated other comprehensive loss	1,955	4,322	5,070	9,375
Total other comprehensive income	1,995	4,162	4,689	9,273
Ending balance	(18,369)	(40,319)	(18,369)	(40,319)
Pension and other postretirement benefits
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(54,538)	(63,450)	(54,995)	(64,416)
OCI (loss) before reclassifications	(2,072)	(1,635)	(2,072)	(1,635)
Amounts reclassified from accumulated other comprehensive loss	762	947	1,219	1,913
Total other comprehensive income	(1,310)	(688)	(853)	278
Ending balance	$ (55,848)	$ (64,138)	$ (55,848)	$ (64,138)