ACQUISITIONS - White Oak - Pro Forma (Details) - USD ($) $ in Thousands		3 Months Ended								5 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Acquisitions, additional information
Acquisition gain, net						$ 22,500						$ 22,548
Pro forma condensed consolidated income statement
Total revenues, As reported		$ 527,291	$ 551,966	$ 439,041	$ 412,725	$ 542,050	$ 566,342	$ 604,619	$ 560,300		$ 1,931,023	2,273,311	[1]	$ 2,300,324	[1]
Net income, As reported											$ 336,553	304,158		492,669
White Oak
Acquisitions, additional information
Acquisition gain, net	$ 22,500
Revenue of acquired business reflected in consolidated statements of income										$ 75,251
Net income of acquired business included in consolidated statements of income										$ 20,687
Pro forma condensed consolidated income statement
Total revenues, Pro forma												2,336,958		2,346,461
Net income, Pro forma												$ 293,206		$ 475,747

[1] Revenues included in the Other and Corporate column are primarily attributable to the Matrix Group revenues, Mt. Vernon transloading revenues, administrative service revenues from affiliates, MAC revenues, Wildcat Insurance revenues and brokerage coal sales.