Investments - Investments Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 118,110	$ 177,280
Gross Unrealized Gains	834	5,967
Gross Unrealized Losses	3,207	1,886
Fair Value	115,737	181,361
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,892
Gross Unrealized Gains	0
Gross Unrealized Losses	22
Fair Value	9,870
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,781	135,590
Gross Unrealized Gains	411	5,311
Gross Unrealized Losses	1,817	1,769
Fair Value	66,375	139,132
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35,955	36,048
Gross Unrealized Gains	423	656
Gross Unrealized Losses	1,355	111
Fair Value	35,023	36,593
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,628	171,638
Gross Unrealized Gains	834	5,967
Gross Unrealized Losses	3,194	1,880
Fair Value	111,268	175,725
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,264	4,232
Gross Unrealized Gains	0	0
Gross Unrealized Losses	13	6
Fair Value	4,251	4,226
Money market fund
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	218	1,410
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	218	1,410
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,482	5,642
Gross Unrealized Gains	0	0
Gross Unrealized Losses	13	6
Fair Value	$ 4,469	$ 5,636