Related-Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related party balances and activity
Expenses recorded for benefits provided by related party	$ 500	$ 500	$ 400
Accounts payable to affiliates	100	100
Services rendered and rent for office space provided
Related party balances and activity
Amounts allocated for services rendered and rent for office space	$ 3,400	2,000	$ 1,700
Affiliated Entity | Services rendered and rent for office space provided
Related party balances and activity
General payment period	30 days
Accounts receivable from affiliates	$ 0	$ 0