Leases Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Alliant Energy and WPL [Member]	Dec. 31, 2005 Sheboygan Falls Energy Facility [Member] WPL [Member] lease_renewal_period_option	Dec. 31, 2012 Synthetic Leases [Member] Alliant Energy and WPL [Member]
Leases [Line Items]
Residual value of assets			$ 4
Operating synthetic leases, remaining term	3 years
Capital lease, agreement term		20 years
Capital leases, number of lease renewal period options		2
Affiliated lease agreement monthly payments		$ 1.3
Basis for lease payments, debt to capital ratio		50.00%
Basis for lease payments, return on equity ratio		10.90%
Approved agreement terms, review period from date of the order		5 years
Capital lease asset amortization term		20 years