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Related Party Transactions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transactions
Receivable from related party	$ 361		$ 361		$ 169
USAA
Related Party Transactions
Receivable from related party	400		400		200
Due to Related Parties	5,900		5,900		$ 3,300
USAA \| Sales and marketing
Related Party Transactions
Costs under related party agreements	$ 6,500	$ 4,200	$ 16,300	$ 12,800