DEBT (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Summary of Convertible Debt

The following table summarizes information regarding the Company’s convertible debt at December 31:

	2017			2016
Convertible Notes due 2017	$	—		$	22,503
Unamortized deferred offering costs		—			(25	)
Convertible Notes due 2017, net		—			22,478
Convertible Notes due 2018		374,980			374,980
Unamortized discount		(12,488	)		(27,566	)
Unamortized deferred offering costs		(1,543	)		(3,484	)
Convertible Notes due 2018, net		360,949			343,930
Convertible Notes due 2020		374,993			374,993
Unamortized discount		(40,287	)		(53,239	)
Unamortized deferred offering costs		(3,631	)		(4,923	)
Convertible Notes due 2020, net		331,075			316,831
Convertible Notes due in 2024		495,000			—
Unamortized discount		(9,355	)		—
Unamortized deferred offering costs		(3,199	)		—
Convertible Notes due in 2024, net		482,446			—
Total convertible debt, net	$	1,174,470		$	683,239
Fair value of fixed rate convertible debt
Convertible Notes due in 2017 (1)	$	—		$	90,977
Convertible Notes due in 2018 (1)		403,955			423,202
Convertible Notes due in 2020 (1)		446,470			442,754
Convertible Notes due in 2024 (1)		493,894			—
Total	$	1,344,319		$	956,933

(1) The fair value of the Company’s fixed rate convertible debt is based on open market trades and is classified as Level 1 in the fair value hierarchy.

Summary of Interest Expense on Debt

Interest expense on the Company’s debt consisted of the following:

	Years Ended December 31,
	2017		2016		2015
Coupon interest	$	10,407	$	9,555	$	9,750
Amortization of debt issuance costs		3,725		3,367		3,294
Accretion of discount on convertible notes		28,575		26,577		25,200
Total interest expense on convertible debt	$	42,707	$	39,499	$	38,244