Debt - Exchangeable Senior Notes (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Aug. 31, 2014 USD ($) $ / shares
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 1,000,000	$ 1,300,000	$ 2,000,000	$ 2,600,000
Convertible Debt | Exchangeable Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Face amount					$ 250,000,000.0
Stated rate (as a percent)	2.63%		2.63%		2.625%
Debt conversion ratio			0.0518215
Conversion price (in USD per share) | $ / shares					$ 19.27
Debt interest expense	$ 2,500,000	2,500,000	$ 4,900,000	4,900,000
Contractual interest expense	1,700,000	1,700,000	3,300,000	3,300,000
Accretion of debt discount	600,000	600,000	1,300,000	1,300,000
Amortization of deferred financing costs	$ 200,000	$ 200,000	$ 300,000	$ 300,000