Investments - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from sale of available for sale securities	$ 8,100	$ 6,600
Gain (loss) in fair value of equity securities included in net income	297	0
Deposits held in trust accounts	7,800		$ 8,200
Deposits, held in trust for collateral requirements	29,400		$ 18,400
Change in fair value of equity securities	$ (297)	$ 0