Summary of Significant Accounting Policies - Leases (Details)	12 Months Ended
	Dec. 31, 2017
Operating Leased Assets [Line Items]
Short-term office lease, maximum term	1 year
Minimum
Operating Leased Assets [Line Items]
Term of office leases	3 years
Maximum
Operating Leased Assets [Line Items]
Term of office leases	5 years