Summary of Significant Accounting Policies - Taxes (Details) - TEXAS - State and Local Jurisdiction [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Applicable tax rate used to determine state margin tax	1.00%
Standard deduction rate used to determine state margin tax	30.00%
Margin tax provision recognized	$ 0.4	$ 0.4	$ 0.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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