Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 53,907	$ 68,742
Less Than 12 Months, Unrealized Losses	2,905	1,416
12 Months or Greater, Fair Value	28,311	7,124
12 Months or Greater, Unrealized Losses	515	470
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	42,178	64,516
Less Than 12 Months, Unrealized Losses	2,034	1,410
12 Months or Greater, Fair Value	28,142	6,971
12 Months or Greater, Unrealized Losses	488	359
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	11,729	0
Less Than 12 Months, Unrealized Losses	871	0
12 Months or Greater, Fair Value	169	153
12 Months or Greater, Unrealized Losses	27	111
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	4,226
Less Than 12 Months, Unrealized Losses	0	6
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	$ 0	$ 0