Rollforward of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012	$ (8,311)
Other comprehensive income (loss) before reclassifications	(6,767)
Amounts reclassified from accumulated other comprehensive income (loss)	(13,186)
Change in accumulated other comprehensive income (loss)	(19,953)
Balance as of September 30, 2013	(28,264)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012	44,196
Other comprehensive income (loss) before reclassifications	(10,924)
Amounts reclassified from accumulated other comprehensive income (loss)	(17,907)
Change in accumulated other comprehensive income (loss)	(28,831)
Balance as of September 30, 2013	15,365
Pension and Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012	(52,507)
Other comprehensive income (loss) before reclassifications	4,157
Amounts reclassified from accumulated other comprehensive income (loss)	4,721
Change in accumulated other comprehensive income (loss)	8,878
Balance as of September 30, 2013	$ (43,629)