Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 93,529	$ 68,742
Less Than 12 Months, Unrealized Losses	2,583	1,416
12 Months or Greater, Fair Value	9,689	7,124
12 Months or Greater, Unrealized Losses	911	470
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	24,923	0
Less Than 12 Months, Unrealized Losses	7	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	49,758	64,516
Less Than 12 Months, Unrealized Losses	1,394	1,410
12 Months or Greater, Fair Value	9,542	6,971
12 Months or Greater, Unrealized Losses	863	359
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	11,544	0
Less Than 12 Months, Unrealized Losses	1,055	0
12 Months or Greater, Fair Value	147	153
12 Months or Greater, Unrealized Losses	48	111
Common stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	3,054	0
Less Than 12 Months, Unrealized Losses	123	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Short-term bond
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	4,250	4,226
Less Than 12 Months, Unrealized Losses	4	6
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	$ 0	$ 0