Summary of Significant Accounting Policies - Revenue and Expense Recognition (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) claim	Dec. 31, 2016 claim	Dec. 31, 2015 claim
Accounting Policies [Abstract]
Minimum amount of insurance coverage	$ 2,000
Maximum amount of insurance coverage	10,000
Average amount of insurance coverage	$ 2,800
Number of claims made | claim	5,671	4,055	3,959