ACQUISITIONS - White Oak - Pro Forma (Details) - USD ($) $ in Thousands		3 Months Ended								5 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquisitions, additional information
Acquisition gain, net													$ 22,548
Pro forma condensed consolidated income statement
Total revenues, As reported		$ 483,130	$ 453,087	$ 398,619	$ 461,005	$ 527,291	$ 551,966	$ 439,041	$ 412,725		$ 1,795,841	$ 1,931,023	2,273,311
Net income, As reported											$ 302,258	$ 336,553	304,158
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, As reported													2,273,311
Total revenues, Pro forma													2,336,958
Net income, As reported													304,158
Net income, Pro forma													$ 293,206