Related Party (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Related Party Transactions
Rent expense on office space leased from Exchange		$ 14.3
Reimbursements from the Exchange related to rent expense		$ 4.9
Estimated rent expense for fiscal year
Related Party Transactions
Rent expense on office space leased from Exchange	$ 17.4