Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net (loss) income	$ (1,002.8)	$ 41.6	$ 207.8	$ 178.3	$ 194.0	$ 281.4	$ 292.2	$ 178.7	$ (575.1)	$ 946.3	$ 802.2
Net change in unrealized gains (losses) on available-for-sale securities (net of $4.0 tax provision for 2012 and $3.9 tax benefit for 2011)
Unrealized holding losses on available-for-sale securities									(15.5)	(5.8)	0
Less: Reclassification for realized losses (gains) included in net income									22.5	(0.9)	0
Net change in unrealized gains (losses) on available-for-sale securities									7.0	(6.7)	0
Net unrealized gains on cash flow hedges (net of $3.9 and $6.2 tax benefit for 2012 and 2011, respectively and $129.5 tax provision for 2010)
Increase in fair value of cash flow hedges									350.4	291.9	229.9
Less: Reclassification for realized gains included in net income									(298.6)	(251.0)	(102.4)
Net unrealized gains on cash flow hedges									51.8	40.9	127.5
Postretirement plans and workers' compensation obligations (net of $43.9 tax provision for 2012 and $63.4 and $2.1 tax benefit for 2011 and 2010, respectively)
Prior service cost (credit) for the period									20.1	0.9	(4.9)
Net actuarial loss for the period									0	(150.1)	(41.3)
Amortization of actuarial loss and prior service cost									55.4	40.5	34.3
Postretirement plan and worker's compensation obligations									75.5	(108.7)	(11.9)
Foreign currency translation adjustment									19.1	0	0
Other comprehensive income (loss), net of income taxes									153.4	(74.5)	115.6
Comprehensive (loss) income									(421.7)	871.8	917.8
Less: Comprehensive income (loss) attributable to noncontrolling interests									10.6	(11.4)	28.2
Comprehensive (loss) income attributable to common stockholders									$ (432.3)	$ 883.2	$ 889.6