CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
NET INCOME	$ 139,598	$ 130,111	$ 172,434	$ 129,630
Derivative instruments:
Net unrealized gain (loss), net of tax benefit (expense) of $105 and $(1,781) for three months ended June 30, 2013 and 2012 and of $67 and $14,770 for six months ended June 30, 2013 and 2012	(160)	2,728	(102)	(22,624)
Reclassification of net realized loss, net of tax benefit of $2,824 and $9,090 for three months ended June 30, 2013 and 2012 and $6,124 and $14,818 for six months ended June 30, 2013 and 2012	4,322	13,925	9,375	22,697
Pension and other postretirement benefits activity, net of tax benefit (expense) of $449 and $(526) for three months ended June 30, 2013 and 2012 and $(182) and $(1,157) for six months ended June 30, 2013 and 2012	(688)	806	278	1,772
Total other comprehensive income	3,474	17,459	9,551	1,845
COMPREHENSIVE INCOME	143,072	147,570	181,985	131,475
Less: Comprehensive income attributable to noncontrolling interests	8,391	7,766	16,783	15,542
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 134,681	$ 139,804	$ 165,202	$ 115,933