Fair Value Measurement (Condensed Consolidated Balance Sheets For Derivative Financial Instruments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Current Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (0.4)	[1]	$ (1.2)	[2]	$ (0.7)	[3]	$ (0.1)	[4]
Net realized and unrealized gains (losses) included in earnings			(2.1)	[5]	0.8	[3],[5]	(0.7)	[4],[5]
Net realized and unrealized (losses) gains included in earnings	0.1	[1],[5]
Transfers into Level 3		[1],[6]		[2],[6]
Transfers out of Level 3		[1],[6]		[2],[6]
Settlements	0.1	[1]	2.6	[2]	0.4	[3]	0.1	[4]
Purchases					(0.7)	[3]
Ending balance	(0.2)	[1],[3]	(0.7)	[2],[4]	(0.2)	[1],[3]	(0.7)	[2],[4]
Net unrealized gains (losses) still held included in earnings	(0.1)	[1],[5]	(0.3)	[2],[5]	0.5	[3],[5]	(0.7)	[4],[5]
Long-Term Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(0.3)	[1]	(0.3)	[2]	(0.3)	[3]	(0.5)	[4]
Net realized and unrealized gains (losses) included in earnings			0.3	[5]	0.1	[3],[5]	0.5	[4],[5]
Net realized and unrealized (losses) gains included in earnings	0.1	[1],[5]
Transfers into Level 3		[1],[6]		[2],[6]
Transfers out of Level 3		[1],[6]		[2],[6]
Settlements		[1]		[2]
Purchases						[3]
Ending balance	(0.2)	[1],[3]			(0.2)	[1],[3]
Net unrealized gains (losses) still held included in earnings	0.1	[1],[5]	0.3	[2],[5]	0.1	[3],[5]	0.3	[4],[5]
Current Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	43.6	[1]	0.6	[2]	1.1	[3]	0.3	[4]
Net realized and unrealized gains (losses) included in earnings			1.2	[2],[5]	9.2	[3],[5]	1.4	[4],[5]
Net realized and unrealized (losses) gains included in earnings	(2.2)	[1],[5]
Transfers out of Level 3	(13.3)	[1],[6]
Settlements	(6.9)	[1]	(0.4)	[2]	(1.9)	[3]	(0.3)	[4]
Purchases					12.8	[3]
Ending balance	21.2	[1],[3]	1.4	[2],[4]	21.2	[1],[3]	1.4	[2],[4]
Net unrealized gains (losses) still held included in earnings	2.6	[1],[5]	1.0	[2],[5]	(8.2)	[3],[5]	1.4	[4],[5]
Long-Term Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	35.5	[1]	0.3	[2]	1.0	[3]	0.3	[4]
Net realized and unrealized gains (losses) included in earnings			2.4	[2],[5]	1.6	[3],[5]	1.0	[4],[5]
Net realized and unrealized (losses) gains included in earnings	(6.2)	[1],[5]
Transfers out of Level 3			(2.5)	[2],[6]			(1.1)	[4],[6]
Settlements		[1]		[2]
Purchases					26.7	[3]
Ending balance	29.3	[1],[3]	0.2	[2],[4]	29.3	[1],[3]	0.2	[2],[4]
Net unrealized gains (losses) still held included in earnings	$ (6.2)	[1],[5]			$ (1.6)	[3],[5]	$ (0.1)	[4],[5]

[1] There were no purchases, issuances and sales of derivatives for the three months ended September 30, 2012.

[2] There were no purchases, issuances and sales of derivatives for the three months ended September 30, 2011.

[3] There were no issuances and sales of derivatives for the nine months ended September 30, 2012.

[4] There were no purchases, issuances and sales of derivatives for the nine months ended September 30, 2011.

[5] Represents the amount of total gains or losses for the period, included in gains or losses from commodity derivative activity, net, attributable to changes in unrealized gains or losses relating to assets and liabilities classified as Level 3.

[6] Amounts transferred in and amounts transferred out are reflected at fair value as of the end of the period.