Related Party Transactions - Transactions with USAA (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Amounts due from related party	$ 169	$ 393
Related party payables	3,200	2,925
Related party accrued expense and other current liabilities	52	992
USAA
Related Party Transaction [Line Items]
Amounts due from related party	200	400
Total amounts due	3,300	3,900
Related party payables	3,200	2,900
Related party accrued expense and other current liabilities	100	1,000
USAA | Selling and Marketing Expense
Related Party Transaction [Line Items]
Sales and marketing expense	$ 16,500	$ 13,900	$ 14,000