Significant Accounting Policies, Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance				$ 365,280
Net current period other comprehensive income (loss)		$ 600	$ 88	(875)	$ (14)
Ending balance		817,436		817,436
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(33,234)	(30,460)	(31,759)	(30,358)
Ending balance		(32,634)	(30,372)	(32,634)	(30,372)
Unrealized Gains (Losses) on Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other comprehensive income (loss) before reclassifications	[1]	563	130	(967)	396
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		$ 37	$ (42)	$ 92	$ (36)

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