Summary of Significant Accounting Policies - Revenue and Expense Recognition (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) claim	Dec. 31, 2017 claim	Dec. 31, 2016 claim
Disaggregation of Revenue [Line Items]
Minimum amount of insurance coverage	$ 2,000
Maximum amount of insurance coverage	10,000
Average amount of insurance coverage	$ 2,900
Number of claims made | claim	7,870	5,671	4,055
Management Services Agreements (MSA) | Minimum
Disaggregation of Revenue [Line Items]
Contract term	3 years
Management Services Agreements (MSA) | Maximum
Disaggregation of Revenue [Line Items]
Contract term	5 years