Rollforward of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012	$ (8,311)
Other comprehensive income (loss) before reclassifications	(26,818)
Amounts reclassified from accumulated other comprehensive income (loss)	(8,646)
Change in accumulated other comprehensive income (loss)	(35,464)
Balance as of March 31, 2013	(43,775)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012	44,196
Other comprehensive income (loss) before reclassifications	(26,818)
Amounts reclassified from accumulated other comprehensive income (loss)	(10,454)
Change in accumulated other comprehensive income (loss)	(37,272)
Balance as of March 31, 2013	6,924
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	0
Amounts reclassified from accumulated other comprehensive income (loss)	1,808
Change in accumulated other comprehensive income (loss)	1,808
Balance as of March 31, 2013	$ (50,699)