Debt - Exchangeable Senior Notes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Aug. 31, 2014
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 900,000	$ 1,100,000	$ 2,900,000	$ 3,100,000
Convertible Debt | Exchangeable Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Face amount					$ 250,000,000.0
Stated rate (as a percent)	0.00%		0.00%		2.625%
Interest expense related to Senior Notes	$ 1,200,000	2,400,000	$ 6,100,000	7,300,000
Contractual interest expense	800,000	1,600,000	4,100,000	4,900,000
Additional non-cash interest expense	300,000	700,000	1,600,000	2,000,000.0
Amortization of deferred financing costs	$ 100,000	$ 100,000	$ 400,000	$ 400,000