Long-Term Debt (Narrative) (Details) $ in Thousands		9 Months Ended
	Mar. 16, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 15, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Credit facility amount outstanding		$ 0
Unamortized debt issuance costs				$ 24
Prosperity Bank [Member]
Debt Instrument [Line Items]
Credit facility maturity date		Jul. 31, 2018
Rate above prime		0.50%
Interest rate	4.50%
Credit facility maximum borrowing capacity		$ 40,000
Credit facility current borrowing capacity	$ 1,250	1,250	$ 3,000
Prosperity Bank [Member] | Loans And Letters Of Credit [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 40,000
Maximum [Member]
Debt Instrument [Line Items]
Interest rate per annum		8.25%
Maximum [Member] | Prosperity Bank [Member]
Debt Instrument [Line Items]
Funded debt to EBITDA	3.5
Minimum [Member]
Debt Instrument [Line Items]
Interest rate per annum		5.50%
Minimum [Member] | Prosperity Bank [Member]
Debt Instrument [Line Items]
Current ratio	1
Interest coverage	3.0