SEGMENTS (Summary of Assets by Segment and Reconciliation of Segment Assets to Total Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Reconciliation of total segment assets to total assets
Assets	$ 6,737,986	$ 6,729,064
Operating Segments
Reconciliation of total segment assets to total assets
Assets	6,210,311	5,999,879
Operating Segments | MPC
Reconciliation of total segment assets to total assets
Assets	2,000,369	1,999,090
Operating Segments | Operating
Reconciliation of total segment assets to total assets
Assets	2,585,774	2,489,177
Operating Segments | Strategic
Reconciliation of total segment assets to total assets
Assets	1,624,168	1,511,612
Corporate and other
Reconciliation of total segment assets to total assets
Assets	$ 527,675	$ 729,185