Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 52,481	$ 68,742
Less Than 12 Months, Unrealized Losses	752	1,416
12 Months or Greater, Fair Value	8,594	7,124
12 Months or Greater, Unrealized Losses	1,003	470
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	45,034	64,516
Less Than 12 Months, Unrealized Losses	480	1,410
12 Months or Greater, Fair Value	8,462	6,971
12 Months or Greater, Unrealized Losses	940	359
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	7,447	0
Less Than 12 Months, Unrealized Losses	272	0
12 Months or Greater, Fair Value	132	153
12 Months or Greater, Unrealized Losses	63	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