COMMITMENTS AND CONTINGENCIES (Narrative) (Details) $ in Millions	1 Months Ended			12 Months Ended
	Jul. 20, 2016 USD ($)	Jun. 30, 2015 USD ($) a	May 31, 2015 USD ($) a	Dec. 31, 2016 USD ($) bbl	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2015 USD ($)	Sep. 09, 2014 bbl	Feb. 25, 2014 bbl	Mar. 31, 2013 USD ($)	Sep. 09, 2010 bbl	Jul. 26, 2010 Miles bbl
Site Contingency [Line Items]
Remaining liabilities				$ 99.9	$ 95.8	$ 6.3
Accrual for environmental loss contingencies				50.8	64.0
Government fines and penalties	$ 61.0
Aggregate insurance coverage limit				650.0
Insurance recoveries, since inception				547.0
Remaining aggregate insurance coverage										$ 145.0
Remaining expected insurance recoveries				$ 103.0			$ 85.0
Natural gas volumes purchased on contractual basis (in percentage)				40.00%
Rent expense				$ 13.0	14.9	18.2
Estimated total loss of crude oil release				$ 1,215.0
Natural gas volumes in custody (in barrels) | bbl				30.0
Natural gas volumes in custody (in barrels) | bbl				66.1
Additional Insurance Recoveries				$ 10.0
Rights Of Way [Member]
Site Contingency [Line Items]
Rent expense				2.6	2.7	3.6
Disputes with Insurers for Which Partial Payment Was Received [Member]
Site Contingency [Line Items]
Insurance recoveries				42.0
Dispute with One Particular Insurer [Member]
Site Contingency [Line Items]
Remaining expected insurance recoveries				85.0
Disputes with Remaining Insurers [Member]
Site Contingency [Line Items]
Remaining expected insurance recoveries				18.0
Liability Program [Member]
Site Contingency [Line Items]
Aggregate insurance coverage limit				900.0
Line 6A Crude Oil Release [Member]
Site Contingency [Line Items]
Government fines and penalties	1.0
Consent Decree [Member]
Site Contingency [Line Items]
Accrual for environmental loss contingencies	110.0
Civil Penalties [Member]
Site Contingency [Line Items]
Government fines and penalties	$ 62.0
Line Six Marshall Leak [Member]
Site Contingency [Line Items]
Remaining liabilities				138.8	149.8
Crude oil released (in barrels) | bbl												20,000
Area Impacted By Crude Oil Release | Miles												38
Costs paid associated with crude oil release				20.8	37.2	$ 141.4
Government fines and penalties				$ 68.5
Loss contingency, pending claims (in claims)				2
Estimated total loss of crude oil release				$ 1.2
Accrual for Environmental Loss Contingencies, Period Increase (Decrease), Total					15.0
Line Six Marshall Leak [Member] | Civil Penalties Related to Clean Water Act [Member]
Site Contingency [Line Items]
Government fines and penalties				61.0
Line Six Marshall Leak [Member] | May 2015 Settlement [Member]
Site Contingency [Line Items]
Costs paid associated with crude oil release			$ 5.0
Area of remediation site (in acres) | a			300
Line Six Marshall Leak [Member] | June 2015 Agreement [Member]
Site Contingency [Line Items]
Accrual for environmental loss contingencies		$ 3.9
Area of remediation site (in acres) | a		175
Line Six Romeoville Leak [Member]
Site Contingency [Line Items]
Crude oil released (in barrels) | bbl								9,000
Costs paid associated with crude oil release					$ 0.6
Estimated total loss of crude oil release				$ 53.0
Crude oil recovered (in barrels) | bbl											1,400
Griffith Terminal Leak [Member]
Site Contingency [Line Items]
Crude oil released (in barrels) | bbl									975