INCOME TAXES (BENEFITS) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Current and other [Abstract]
Federal	$ (1.3)	$ (15.4)	$ (34.1)
State	0.1	1.0	0
Total current and other	(1.2)	(14.4)	(34.1)
Deferred [Abstract]
Federal	91.7	132.7	114.1
State	(0.1)	(0.1)	0.5
Total deferred	91.6	132.6	114.6
Amortization of excess deferred taxes	0.4	1.1	1.7
Investment tax credits	3.1	3.3	3.3
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	34.1	35.7	38.2	93.9
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	5.6	4.8	4.5
SPPC [Member]
Current and other [Abstract]
Federal	(0.1)	1.1	(0.5)
State	0.1	0.9	0
Total current and other	0	2.0	(0.5)
Deferred [Abstract]
Federal	33.2	42.0	34.3
State	0.2	(0.9)	0.3
Total deferred	33.4	41.1	34.6
Amortization of excess deferred taxes	0.3	0.8	0.8
Investment tax credits	1.9	1.9	2.1
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	9.8	10.2	10.5	40.1
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	1.8	1.6	1.4
NPC [Member]
Current and other [Abstract]
Federal	(1.1)	(0.9)	(34.3)
State	0	0
Total current and other	(1.1)	(0.9)	(34.3)
Deferred [Abstract]
Federal	73.4	93.6	97.9
State	(0.3)	0.7	0.3
Total deferred	73.1	94.3	98.2
Amortization of excess deferred taxes	0.1	0.2	0.9
Investment tax credits	1.2	1.4	1.3
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	24.3	25.5	26.6	48.5
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	$ 3.8	$ 3.2	$ 3.1