Summary of Significant Accounting Policies - Concentration Risk (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) customer	Dec. 31, 2017 USD ($) customer
Concentrations of risk
Accrued interest \| $	$ 2.3	$ 1.7
Amounts on deposit in excess of federally insured limits approximately \| $	$ 26.0	$ 16.3
Payables \| Service providers or vendors
Concentrations of risk
Number of service providers \| customer	4	2
Concentration risk percentage	49.00%	40.00%
Number of critical suppliers \| customer	3

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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