Changes in Accumulated Other Comprehensive Loss (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	$ (114,008)
OCI (loss) before reclassifications	5,381
Amounts reclassified from accumulated other comprehensive loss	30,574
Other comprehensive income (loss) attributable to common shareholders	35,955	38,155	7,605
Ending balance	(78,053)	(114,008)
Derivative Instruments
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(49,592)
OCI (loss) before reclassifications	(213)
Amounts reclassified from accumulated other comprehensive loss	26,747
Other comprehensive income (loss) attributable to common shareholders	26,534
Ending balance	(23,058)
Pension and other postretirement benefits
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(64,416)
OCI (loss) before reclassifications	5,594
Amounts reclassified from accumulated other comprehensive loss	3,827
Other comprehensive income (loss) attributable to common shareholders	9,421
Ending balance	$ (54,995)