COMMON EQUIY - CAPITAL TRANSACTIONS WITH SUBSIDIARIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Capital Transactions with Subsidiaries
Dividends to parent	$ 186.1
Return of capital to parent	52.5
Equity contributions from parent	333.7
IBS
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	0
Equity contributions from parent	25.0
ITF
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	0
Equity contributions from parent	50.3
MERC
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	27.0
Equity contributions from parent	20.0
MGU
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	13.0
Equity contributions from parent	7.0
PGL
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	0
Equity contributions from parent	65.0
UPPCO
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	12.5
Equity contributions from parent	94.4
WPS
Capital Transactions with Subsidiaries
Dividends to parent	111.8
Return of capital to parent	0
Equity contributions from parent	55.0
WPS Investments, LLC
Capital Transactions with Subsidiaries
Dividends to parent	74.3
Return of capital to parent	0
Equity contributions from parent	$ 17.0
WPS Investments, LLC
Capital Transactions with Subsidiaries
Ownership interest (as a percent)	89.02%
WPS Investments, LLC | WPS
Capital Transactions with Subsidiaries
Ownership interest (as a percent)	10.98%