Debt - Exchangeable Senior Notes (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Aug. 31, 2014 USD ($) $ / shares
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 1,100,000	$ 1,000,000	$ 3,100,000	$ 3,600,000
Convertible Debt | Exchangeable Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Face amount					$ 250,000,000.0
Stated rate (as a percent)	2.625%		2.625%		2.625%
Debt conversion ratio			0.0519443
Conversion price (in USD per share) | $ / shares					$ 19.25
Debt interest expense	$ 2,400,000	2,400,000	$ 7,300,000	7,300,000
Contractual interest expense	1,600,000	1,600,000	4,900,000	4,900,000
Accretion of debt discount	700,000	700,000	2,000,000	2,000,000
Amortization of deferred financing costs	$ 100,000	$ 100,000	$ 400,000	$ 400,000