COMMON EQUITY - CAPITAL TRANSACTIONS WITH SUBSIDIARIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Capital Transactions with Subsidiaries
Dividends to parent	$ 50.0
Return of capital to parent	23.5
Equity contributions from parent	2.7
WPS Investments, LLC
Capital Transactions with Subsidiaries
Ownership interest (as a percent)	89.06%
ITF
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	0
Equity contributions from parent	1.0
MERC
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	10.5
Equity contributions from parent	0
MGU
Capital Transactions with Subsidiaries
Dividends to parent	0
Return of capital to parent	13.0
Equity contributions from parent	0
NSG
Capital Transactions with Subsidiaries
Dividends to parent	7.5
Return of capital to parent	0
Equity contributions from parent	0
WPS
Capital Transactions with Subsidiaries
Dividends to parent	28.8
Return of capital to parent	0
Equity contributions from parent	0
WPS | WPS Investments, LLC
Capital Transactions with Subsidiaries
Ownership interest (as a percent)	10.94%
WPS Investments, LLC
Capital Transactions with Subsidiaries
Dividends to parent	13.7
Return of capital to parent	0
Equity contributions from parent	$ 1.7