Equity (Details) - USD ($)			1 Months Ended	2 Months Ended	12 Months Ended
		Dec. 05, 2018	Jan. 31, 2019	Jan. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016		Feb. 20, 2018
Class of Stock [Line Items]
Common stock, par value (in dollars per share)					$ 0.001		$ 0.001
Common stock authorized (shares)					250,000,000
Common stock authorized for repurchase					$ 155,700,000						$ 500,000,000.0
Stock repurchased during period (shares)					3,600,000		5,700,000
Stock repurchased during period					$ 63,100,000		$ 50,000,000
Preferred stock authorized (shares)					4,000,000.0
Preferred stock issued (shares)					0
Preferred stock outstanding (shares)					0		0
Series A preferred shares authorized (shares)					1,000,000.0
Series A preferred shares par value (in usd per share)					$ 0.001
Accretion of unamortized discount and beneficial conversion feature		$ 14,700,000			$ 17,567,000	[1]	$ 3,532,000		$ 3,244,000
Settlement of beneficial conversion feature		6,100,000
Net increase in dividend equivalents on Series A convertible preferred shares	[2]	$ (8,600,000)			11,429,000		3,532,000		3,244,000
Shares repurchased (shares)		100,000
Carrying value of preferred stock		$ 100,000,000			0		0
Aggregate purchase price		183,700,000
Excess over carrying value		$ 83,700,000			108,224,000	[2]	12,000,000	[2]	$ 12,000,000	[2]
Preferred shares converted into common stock (shares)		100,000
Number of shares issued upon conversion (shares)		6,896,548
Inducements paid		$ 12,000,000			$ 12,000,000
Other costs		$ 500,000
Series A preferred shares dividend rate (percent)					6.00%
Accrued payments to induce conversion					$ 3,000,000
Accrued preferred dividends							$ 3,000,000
Subsequent event
Class of Stock [Line Items]
Inducements paid			$ 3,000,000	$ 15,000,000

[1] Represents total accretion of $17.6 million, net of $6.1 million acquired value of beneficial conversion feature attributable to repurchased Series A Convertible Preferred Stock.

[2] On December 5, 2018, all issued and outstanding shares of Series A Convertible Preferred Stock were repurchased in exchange for cash or converted to common stock. As a result, amounts reported for the year ended December 31, 2018, include amounts resulting from the repurchase and conversion, in addition to dividends, payments to induce conversion, and accretion of dividend equivalents prior to December 5, 2018. See Note 1 — Basis of Presentation and Summary of Significant Accounting Policies, for additional information.