Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 151,625	$ 177,280
Gross Unrealized Gain	1,493	5,967
Gross Unrealized Loss	3,420	1,886
Fair Value	149,698	181,361
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,995
Gross Unrealized Gain	1
Gross Unrealized Loss	0
Fair Value	9,996
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	103,251	135,590
Gross Unrealized Gain	778	5,311
Gross Unrealized Loss	2,522	1,769
Fair Value	101,507	139,132
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,923	36,048
Gross Unrealized Gain	708	656
Gross Unrealized Loss	898	111
Fair Value	33,733	36,593
Debt and equity investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147,169	171,638
Gross Unrealized Gain	1,487	5,967
Gross Unrealized Loss	3,420	1,880
Fair Value	145,236	175,725
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,239	4,232
Gross Unrealized Gain	6	0
Gross Unrealized Loss	0	6
Fair Value	4,245	4,226
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	217	1,410
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	217	1,410
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,456	5,642
Gross Unrealized Gain	6	0
Gross Unrealized Loss	0	6
Fair Value	$ 4,462	$ 5,636