(10) DEBT

Convertible Notes

As of June 30, 2018, the Company had outstanding fixed-rate notes with varying maturities for an aggregate principal amount of $1.2 billion (collectively the Notes). The Notes are senior subordinated convertible obligations, and interest is payable in arrears, quarterly. The following table summarizes information regarding the Company’s convertible debt:

	June 30,			December 31,
	2018			2017
0.75% senior subordinated convertible notes due in October 2018	$	374,980		$	374,980
Unamortized discount		(4,659	)		(12,488	)
Unamortized deferred offering costs		(569	)		(1,543	)
Convertible Notes due in 2018, net		369,752			360,949

1.50% senior subordinated convertible notes due in October 2020		374,993			374,993
Unamortized discount		(33,531	)		(40,287	)
Unamortized deferred offering costs		(2,983	)		(3,631	)
Convertible Notes due in 2020, net		338,479			331,075

0.599% senior subordinated convertible notes due in August 2024		495,000			495,000
Unamortized discount		(8,651	)		(9,355	)
Unamortized deferred offering costs		(2,957	)		(3,199	)
Convertible Notes due in 2024, net		483,392			482,446

Total convertible debt, net	$	1,191,623		$	1,174,470

Fair value of fixed rate convertible debt
Convertible Notes due in 2018 ⁽¹⁾	$	398,874		$	403,955
Convertible Notes due in 2020 ⁽¹⁾		442,240			446,470
Convertible Notes due in 2024 ⁽¹⁾		502,509			493,894
Total	$	1,343,623		$	1,344,319

(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. See Note 12 to these Condensed Consolidated Financial Statements for additional information related to the Company’s fair value measurements.

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Coupon interest expense	$	3,527	$	2,194	$	6,488	$	4,558
Amortization of debt issuance costs		1,006		886		2,010		1,772
Accretion of discount on convertible notes		7,692		6,960		15,289		13,829
Total interest expense on convertible debt	$	12,225	$	10,040	$	23,787	$	20,159

See Note 13 to the Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 for additional information related to the Company’s convertible debt.

Revolving Credit Facility

The Company maintains a senior unsecured revolving credit facility (Credit Facility) that provides revolving credit of up to $100.0 million in revolving loans (the Revolving Credit Facility), a $10.0 million letter of credit subfacility and a $15.0 million swingline loan subfacility. The maturity date of the Revolving Credit Facility will occur on November 29, 2018. As of June 30, 2018 and December 31, 2017, there were no outstanding amounts due on nor any usage of the Credit Facility. As of June 30, 2018, the Company and certain of its subsidiaries that serve as guarantors were in compliance with all covenants.