Convertible Senior Notes - Additional Information (Detail)			1 Months Ended				5 Months Ended	6 Months Ended	12 Months Ended
	Dec. 23, 2013 USD ($)	Dec. 17, 2013 USD ($) note	Oct. 31, 2017 USD ($) shares	Jul. 31, 2017 shares	Dec. 31, 2016 USD ($) open_market_purchase shares	Dec. 31, 2013 USD ($) $ / shares	Dec. 31, 2017 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares
Debt Disclosure [Line Items]
Stock issued during period, shares, new issues (shares) | shares				3,243,882			10,314,250			10,890,915
Common shares sold (shares)							$ 100,000	$ 103,000	$ 100,000
2018 Convertible Notes [Member]
Debt Disclosure [Line Items]
Principal							40,565,000	40,565,000	40,565,000
Fair value							74,300,000	$ 81,000,000	74,300,000
2.75% Convertible Senior Notes Due 2018 [Member]
Debt Disclosure [Line Items]
Sale of convertible notes, principal amount		$ 120,000,000
Interest rate		2.75%						2.75%
Number of notes if converted | note		120,000
Debt instrument face value							$ 40,600,000	$ 40,600,000	40,600,000
Conversion rate, shares (shares) | shares								95
Conversion rate, price per share ($ per share) | $ / shares								$ 1,000
Common shares converted value (shares)						$ 11,400,000		$ 3,900,000
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)
Number of open market purchases | open_market_purchase					2
Number of instruments (shares) | shares			69,472		9,963
Repurchase of principal			$ 69,500,000		$ 10,000,000
Repayments			$ 27,300,000		9,000,000
Gain (loss) on retirement									(800,000)	$ (25,000)
Proceeds from unwinding					$ 21,000				$ 5,800,000
Stock issued during period, shares, new issues (shares) | shares			5,400,000
Stock issued, value			$ 73,000,000
Strike price per share ($ per share) | $ / shares						$ 14.03
Common shares sold (shares)						$ 11,400,000