Benefit Plans (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2009	Dec. 31, 2011
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	$ 5
Total plan assets, percentage of common stock (less than 1%)	1.00%		1.00%
Interest crediting rate, percentage above annual change in consumer price index		3.00%
Common stock percentage in assets held saving plans	12.50%		14.60%
Unrecognized compensation cost	6.1
Weighted average remaining contractual term	1 year
Intrinsic value of options outstanding and exercisable	0.5
Percentage of base salary and performance-based compensation	100.00%
Performance Shares and Units [Member]
Benefit Plans [Line Items]
Performance period	3 years
Expected volatility based on historical daily stock prices, historical period	3 years
Performance Shares [Member]
Benefit Plans [Line Items]
Performance multiplier range, lower	0.00%
Performance multiplier range, upper	200.00%
Performance Units [Member]
Benefit Plans [Line Items]
Performance multiplier range, lower	0.00%
Performance multiplier range, upper	200.00%
Non-Qualified Stock Options [Member]
Benefit Plans [Line Items]
Vesting period	3 years
Expiration period	10 years
Minimum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	1 year
Minimum [Member] | Performance-Contingent Restricted Stock [Member]
Benefit Plans [Line Items]
Performance period	2 years
Minimum [Member] | Performance Contingent Cash Awards [Member]
Benefit Plans [Line Items]
Performance period	2 years
Maximum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	2 years
Maximum [Member] | Time-Based Restricted Stock [Member]
Benefit Plans [Line Items]
Vesting period	3 years
Maximum [Member] | Performance-Contingent Restricted Stock [Member]
Benefit Plans [Line Items]
Performance period	4 years
Maximum [Member] | Performance Contingent Cash Awards [Member]
Benefit Plans [Line Items]
Performance period	4 years
Omnibus Incentive Plan [Member]
Benefit Plans [Line Items]
Shares available for issuance under the OIP	4.2
IPL [Member] | Allocated From Corporate Services [Member]
Benefit Plans [Line Items]
Estimated amortization	1.3
WPL [Member] | Allocated From Corporate Services [Member]
Benefit Plans [Line Items]
Estimated amortization	$ 0.9
U.S. Currency [Member]
Benefit Plans [Line Items]
Securities lending, initial collateral levels	102.00%
Other Securities [Member]
Benefit Plans [Line Items]
Securities lending, initial collateral levels	102.00%
Foreign Currency [Member]
Benefit Plans [Line Items]
Securities lending, initial collateral levels	105.00%
Non Affiliated Borrowers [Member] | U.S. Currency [Member]
Benefit Plans [Line Items]
Securities lending, initial collateral levels	100.00%
Affiliated Borrowers [Member] | U.S. Currency [Member]
Benefit Plans [Line Items]
Securities lending, initial collateral levels	102.00%