Rollforward of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012		$ (8,311)
Other comprehensive loss before reclassifications		(10,644)
Amounts reclassified from accumulated other comprehensive income (loss)		(9,699)
Change in accumulated other comprehensive income (loss)		(20,343)
Balance as of June 30, 2013	(28,654)	(28,654)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012		44,196
Other comprehensive loss before reclassifications		(10,644)
Amounts reclassified from accumulated other comprehensive income (loss)		(12,967)
Change in accumulated other comprehensive income (loss)		(23,611)
Balance as of June 30, 2013	20,585	20,585
Pension and Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance as of December 31, 2012		(52,507)
Other comprehensive loss before reclassifications		0
Amounts reclassified from accumulated other comprehensive income (loss)		3,268
Change in accumulated other comprehensive income (loss)	3,268
Balance as of June 30, 2013	$ (49,239)	$ (49,239)