Fair Value of Financial Instruments - Changes in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Convertible Debt, Fair Value Disclosures	$ 600
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	383,598	$ 373,403	$ 357,989	$ 365,239	$ 366,319	$ 343,544	$ 357,989	$ 343,544
Change in net unrealized gain, net of tax	22			69			1,444	(178)
Amounts reclassified for net realized gain included in net income	(18)			154			25	249
Other comprehensive income (loss), net of tax	1,031	930	896	69	144	(391)	2,857	(178)
Ending balance	361,023	$ 383,598	373,403	$ 374,140	$ 365,239	$ 366,319	361,023	$ 374,140
Unrealized Gain (Loss) on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ (545)				(545)
Change in net unrealized gain, net of tax							1,419
Amounts reclassified for net realized gain included in net income							25
Other comprehensive income (loss), net of tax							1,444
Ending balance	$ 899						$ 899