SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash, Cash Equivalents and Restricted Cash and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents				$ 143,694	$ 165,893
Restricted cash and investments - current				1,292	997
Restricted Cash and Investments, Noncurrent				5,979	1,000
Cash, cash equivalents and restricted cash and investments	$ 167,890	$ 155,117	$ 141,628	$ 150,965	167,890
Operating activities
Other	(6,039)	(288)	21,065
Net cash provided by operating activities	240,383	250,812	182,772
Financing activities
Other	(1,349)	(854)
Net cash provided by (used in) financing activities	(44,550)	(18,283)	111,793
Net increase (decrease) in cash, cash equivalents and restricted cash and investments	(16,925)	12,773	13,489
Cash, cash equivalents and restricted cash and investments – beginning	167,890	155,117	141,628
Cash, cash equivalents and restricted cash and investments – ending	150,965	167,890	155,117
Accounting Standards Update 2016-18
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, cash equivalents and restricted cash and investments	167,890	155,117	141,628		167,890
Operating activities
Other		(288)	21,065
Net cash provided by operating activities		250,812	182,772
Financing activities
Other		(854)
Net cash provided by (used in) financing activities		(18,283)	111,793
Net increase (decrease) in cash, cash equivalents and restricted cash and investments		12,773	13,489
Cash, cash equivalents and restricted cash and investments – beginning	167,890	155,117	141,628
Cash, cash equivalents and restricted cash and investments – ending		167,890	155,117
Accounting Standards Update 2016-18 | As Originally Reported
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, cash equivalents and restricted cash and investments	165,893	144,587	92,468		165,893
Operating activities
Change in restricted cash and investments		(137)	29,113
Other		(175)	21,324
Net cash provided by operating activities		250,788	212,144
Financing activities
Proceeds from investment in securities pledged as collateral		7,426	8,931
Other		277	327
Net cash provided by (used in) financing activities		(9,726)	121,051
Net increase (decrease) in cash, cash equivalents and restricted cash and investments		21,306	52,119
Cash, cash equivalents and restricted cash and investments – beginning	165,893	144,587	92,468
Cash, cash equivalents and restricted cash and investments – ending		165,893	144,587
Accounting Standards Update 2016-18 | Effect of Change
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, cash equivalents and restricted cash and investments	1,997	10,530	49,160		$ 1,997
Operating activities
Change in restricted cash and investments		137	(29,113)
Other		(113)	(259)
Net cash provided by operating activities		24	(29,372)
Financing activities
Proceeds from investment in securities pledged as collateral		(7,426)	(8,931)
Other		(1,131)	(327)
Net cash provided by (used in) financing activities		(8,557)	(9,258)
Net increase (decrease) in cash, cash equivalents and restricted cash and investments		(8,533)	(38,630)
Cash, cash equivalents and restricted cash and investments – beginning	$ 1,997	10,530	49,160
Cash, cash equivalents and restricted cash and investments – ending		$ 1,997	$ 10,530