Convertible Senior Notes - Additional Information (Details)		1 Months Ended				5 Months Ended	12 Months Ended
	Dec. 17, 2013 USD ($) shares	Oct. 31, 2017 USD ($) shares	Jul. 31, 2017 shares	Dec. 31, 2016 USD ($) open_market_purchase shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Debt Instrument [Line Items]
Common stock, shares issued (shares) | shares						100,742,735	110,525,141	100,742,735
Payments for repurchase							$ 0	$ 27,500,000	$ 9,014,000
Stock issued (shares) | shares			3,243,882			10,314,250			10,890,915
2018 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument face value						$ 74,300,000		74,300,000
Conversion rate (shares) | shares							95
Conversion rate, price per $1,000 | $ / shares							$ 1,000
Money conversion price per share ($ per share) | $ / shares							$ 10.53
Long-term debt, gross						40,565,000		40,565,000
2.75% Convertible Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Sale of convertible notes, principal amount	$ 120,000,000
Interest rate	2.75%
Debt conversion, converted instrument, securities converted (in shares) | shares	120,000
Debt instrument face value						$ 38,200,000	$ 0	38,200,000
Common shares converted value (shares) | shares		69,472		9,963	11,400,000
Conversion rate (shares) | shares							95
Conversion rate, price per $1,000 | $ / shares							$ 1,000
Money conversion price per share ($ per share) | $ / shares							$ 10.53
Net proceeds from convertible notes					$ 115,400,000
Professional fee					4,600,000
Additional paid-in capital					$ 13,100,000
Strike price per share ($ per share) | $ / shares					$ 14.03
Common stock, shares issued (shares) | shares					11,400,000
Number Of open market purchases | open_market_purchase				2
Repurchase of principal		$ 69,500,000		$ 10,000,000
Payments for repurchase		$ 27,300,000		$ 9,000,000
Loss on 2018 Convertible Note purchase								800,000	$ 25,000
Proceeds from unwinding								$ 5,800,000	$ 21,000
Stock issued (shares) | shares		5,400,000
Common stock issued, value		$ 73,000,000