Accumulated Other Comprehensive Loss (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized losses on cash flow hedges, Beginning Balance	$ (61,547)	$ (59,263)
Unrealized losses on cash flow hedges, Current period change excluding amounts reclassified from other comprehensive income	(8,302)	(6,843)
Unrealized losses on cash flow hedges, Amounts reclassified from accumulated other comprehensive income	4,726	5,065
Unrealized losses on cash flow hedges, Total change in accumulated other comprehensive income	(3,576)	(1,778)
Unrealized losses on cash flow hedges, Ending Balance	(65,123)	(61,041)
Items not yet recognized as a component of net periodic benefit cost (pension and other postretirement plans), Beginning Balance	(21,833)	(10,081)
Items not yet recognized as a component of net periodic benefit cost (pension and other postretirement plans), Current period change excluding amounts reclassified from other comprehensive income	496	(104)
Items not yet recognized as a component of net periodic benefit cost (pension and other postretirement plans), Amounts reclassified from accumulated other comprehensive income	0	0
Items not yet recognized as a component of net periodic benefit cost (pension and other postretirement plans), Total change in accumulated other comprehensive income	496	(104)
Items not yet recognized as a component of net periodic benefit cost (pension and other postretirement plans), Ending Balance	(21,337)	(10,185)
Beginning Balance	(83,380)	(69,344)
Current period change excluding amounts reclassified from other comprehensive income	(7,806)	(6,947)
Amounts reclassified from accumulated other comprehensive income	4,726	5,065
Total change in accumulated other comprehensive income	(3,080)	(1,882)
Ending Balance	$ (86,460)	$ (71,226)