Debt - Exchangeable Senior Notes (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Aug. 31, 2014 USD ($) $ / shares
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 1,000,000	$ 1,300,000
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%		2.625%
Debt conversion ratio	0.0518215
Conversion price (in USD per share) | $ / shares			$ 19.30
Debt interest expense	$ 2,400,000	2,400,000
Contractual interest expense	1,600,000	1,600,000
Accretion of debt discount	700,000	700,000
Amortization of deferred financing costs	$ 100,000	$ 100,000