Significant Accounting Policies, Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance				$ 365,280
Net current period other comprehensive income (loss)		$ 105	$ 173	(773)	$ 159
Ending balance		840,144		840,144
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(32,637)	(30,372)	(31,759)	(30,358)
Ending balance		(32,532)	(30,199)	(32,532)	(30,199)
Unrealized Gains (Losses) on Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other comprehensive income (loss) before reclassifications	[1]	133	215	(834)	610
Amounts reclassified from accumulated other comprehensive loss		0	0	0	(374)
Other comprehensive income (loss), tax		0	0	0	0
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other comprehensive income (loss) before reclassifications		$ (28)	$ (42)	$ 61	$ (77)

[1] There was no income tax expense or benefit related to elements of other comprehensive income (loss) for the three and nine months ended September 30, 2018 and 2017.