Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Benefit Plans [Line Items]
Total plan assets, percentage of common stock (less than 1%)	1.00%	1.00%
Common stock percentage in assets held in 401(k) saving plans	11.50%	12.60%
Unrecognized compensation cost	$ 5.6
Percentage of base salary and performance-based compensation	100.00%
Carrying value of deferred compensation obligations	$ 21.8	$ 19.4
Performance Shares and Performance Units [Member]
Benefit Plans [Line Items]
Performance period	3 years
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Performance Shares [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Performance Restricted Stock Unit And Performance Restricted Unit [Member]
Benefit Plans [Line Items]
Performance period	3 years
Actual number of shares paid out upon vesting, maximum percentage of target shares	200.00%
Actual number of shares paid out upon vesting, minimum percentage of target shares	0.00%
Performance Restricted Stock Unit [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
Restricted Stock Unit And Restricted Unit [Member]
Benefit Plans [Line Items]
Performance period	3 years
Instrument valuation based on shares of common stock, number of shares	1
Minimum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	1 year
Maximum [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost recognized over a weighted average period	2 years
IPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	$ 3.1
WPL [Member]
Benefit Plans [Line Items]
Unrecognized compensation cost	2.2
Other Postretirement Benefits Plans [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5.0
Other Postretirement Benefits Plans [Member] | IPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	5.0
Other Postretirement Benefits Plans [Member] | WPL [Member]
Benefit Plans [Line Items]
Plan asset threshold for long-term allocation targets	$ 5.0
Omnibus Incentive Plan [Member]
Benefit Plans [Line Items]
Shares available for issuance under the Amended and Restated OIP	7,100,000
2015 Grant [Member] | Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1
2016 Grant [Member] | Performance Units [Member]
Benefit Plans [Line Items]
Instrument valuation based on shares of common stock, number of shares	1