SUMMARIZED QUARTERLY FINANCIAL DATA (Unaudited) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 59.4	$ 93.9	$ 263.9	$ 142.7	$ 48.4	$ 104.5	$ 237.7	$ 417.2	$ 390.6	$ 533.3	$ 451.6
Operating income (loss)	(12.1)	1.9	72.4	34.7	(22.5)	2.2	79.3	62.2	59.0	93.7	93.3
Net income (loss)	$ (9.4)	$ (0.6)	$ 43.0	$ 19.8	$ (8.9)	$ (0.7)	$ 47.2	$ 33.0	$ 37.6	$ 57.4	$ 49.4