Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Equity securities, fv-ni, unrealized loss	$ 298
Available-for-sale equity securities, gross unrealized loss		$ 726
Adjusted Cost	157,940		$ 77,592
Unrealized Gains	74		32
Unrealized Losses	(745)		(113)
Fair Value	157,269		77,511
Marketable securities, realized gains	213
Available for sale securities, gross realized gains		89
Marketable securities, realized losses	134
Available for sale securities, gross realized losses		$ 518
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Equity securities, cost	10,461
Equity Securities, accumulated unrealized gain	28
Equity Securities, accumulated unrealized loss	(326)
Equity securities, fair value	10,163
Adjusted Cost			443
Unrealized Gains			31
Unrealized Losses			(6)
Fair Value			468
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	60,073		44,525
Unrealized Gains	0		0
Unrealized Losses	(14)		(44)
Fair Value	60,059		44,481
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	41,554		9,285
Unrealized Gains	4		1
Unrealized Losses	(244)		(24)
Fair Value	41,314		9,262
Government securities - U.S.
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	36,868		19,080
Unrealized Gains	40		0
Unrealized Losses	(131)		(30)
Fair Value	36,777		19,050
Other fixed-income securities
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	8,984		4,259
Unrealized Gains	2		0
Unrealized Losses	(30)		(9)
Fair Value	$ 8,956		$ 4,250