Reclassifications out of accumulated other comprehensive (loss) (Details 16) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reclassification adjustment out of accumulated other comprehensive income
Operating revenues	$ 1,184,423	$ 1,285,782	$ 1,060,595	$ 931,604	$ 1,081,144	$ 1,173,518	$ 967,962	$ 852,807	$ 4,462,404	$ 4,075,431	$ 4,050,492
Interest expense									(83,917)	(76,699)	(81,354)
Earnings (loss) from equity method investments									(132)	5,383	4,693
Income before income taxes									415,618	(45,467)	336,226
Income taxes									136,736	(31,146)	110,274
Net income (loss)									278,570	(754)	213,026
Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income
Income before income taxes									6,737
Income taxes									(2,548)
Net income (loss)									4,189
Amortization of postretirement liability losses included in net periodic benefit cost
Reclassification adjustment out of accumulated other comprehensive income
Net periodic benefit cost									(3,277)
Income taxes									1,276
Net income (loss)									(2,001)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income
Reclassification adjustment out of accumulated other comprehensive income
Earnings (loss) from equity method investments									(213)
Earnings (loss) from equity method investments, tax									70
Net income (loss)									(143)
Reclassification adjustment for loss on available-for-sale investments included in net income [Member]
Reclassification adjustment out of accumulated other comprehensive income
Other income									(168)
Income taxes									59
Net income (loss)									(109)
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income
Net income (loss)									(1,936)
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income
Operating revenues									7,803
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income
Interest expense									$ (1,066)