LONG-TERM DEBT / INTEREST EXPENSE - Convertible Debt: Issuance and Terms (Details)		12 Months Ended
	Aug. 10, 2015 USD ($) day $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Issuance of shares associated with exchange of convertible senior notes | $		$ 0	$ 0	$ 71,670,000
Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount) | Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount)
Debt Instrument [Line Items]
Principal | $	$ 287,500,000	159,097,000	$ 188,565,000
Interest rate	1.50%
Convertible debt, conversion rate (in shares)	0.0151156
Issuance of shares associated with exchange of convertible senior notes | $		$ 1,000
Convertible debt, conversion price (in dollars per share) | $ / shares	$ 66.16
Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount) | Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount) | Redemption Prior to May 15, 2010 | Debt Instrument, Conversion, Option One
Debt Instrument [Line Items]
Convertible debt, trading days threshold | day	20
Convertible debt, consecutive trading days threshold | day	30
Convertible debt, percentage of stock price trigger threshold	130.00%
Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount) | Notes (net of $3.9 million and $11.5 million conversion feature and $0.5 million and $1.6 million discount) | Redemption Prior to May 15, 2010 | Debt Instrument, Conversion, Option Two
Debt Instrument [Line Items]
Convertible debt, consecutive trading days threshold | day	10
Convertible debt, consecutive business days threshold	5 days
Convertible debt, ratio of trading price per $1,000 in principal amount	98.00%