CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
NET INCOME	$ 130,111	$ 93,839	$ 129,630	$ 84,165
Derivative instruments:
Net unrealized gain (loss), net of tax benefit (expense) of ($1,781) and $6,448 for three months ended June 30, 2012 and 2011 and $14,770 and $6,058 for six months ended June 30, 2012 and 2011, respectively	2,728	(9,875)	(22,624)	(9,277)
Reclassification of net realized loss, net of tax benefit of $9,090 and $9,988 for three months ended June 30, 2012 and 2011 and $14,818 and $15,852 for six months ended June 30, 2012 and 2011, respectively	13,925	15,299	22,697	24,281
Pension and other postretirement benefits activity, net of tax expense of $526 and $797 for three months ended June 30, 2012 and 2011 and $1,157 and $1,363 for six months ended June 30, 2012 and 2011, respectively	806	1,222	1,772	2,088
Total other comprehensive income	17,459	6,646	1,845	17,092
COMPREHENSIVE INCOME	147,570	100,485	131,475	101,257
Less: Comprehensive income attributable to noncontrolling interests	7,766	7,154	15,542	12,615
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDER	$ 139,804	$ 93,331	$ 115,933	$ 88,642