Notes Payable - 2.25% Convertible Senior Notes due 2022 - General Information (Details)	1 Months Ended	3 Months Ended
	Jun. 30, 2015 USD ($) D $ / shares shares	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Notes Payable
Net proceed received			$ 146,250,000
Payments for convertible note hedges	$ 21,100,000
2.25% Convertible Senior Notes due in 2022 | Convertible Senior Notes
Notes Payable
Aggregate principal amount of notes issued	335,700,000	$ 335,699,000		$ 335,699,000
Net proceed received	324,000,000
Fees and expenses	$ 11,700,000
Annual interest rate of notes (as a percent)	2.25%	2.25%
Initial conversion rate, number of shares to be issued per $1000 of principal amount (in shares)	60.3209
Principal amount used for debt instrument conversion ratio	$ 1,000
Initial conversion price (in dollars per share) | $ / shares	$ 16.58
Shares issuable upon conversion of debt (in shares) | shares	20,249,665
Number of trading days | D	20
Consecutive trading days | D	30
Minimum percentage of stock price	130.00%
Number of business days immediately after any five consecutive trading day period during the measurement period | D	5
Number of consecutive trading days before five business days during the measurement period | D	5
Percentage of the trading price to the product of the sale price of the entity's common stock and the conversion rate	98.00%
Repurchase price, as percentage of principal amount, if company undergoes change of control	100.00%
Percentage of aggregate principal amount payable, in case of event of default	25.00%
Debt issuance costs capitalized		$ 5,750,000		$ 5,976,000
Maximum period of the sole remedy for event failures in the Indenture		180 days
Amortization period	7 years