Long-Term Debt (Narrative) (Details) - Prosperity Bank [Member]		3 Months Ended
	Mar. 21, 2018 USD ($)	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Credit facility maturity date		Jul. 31, 2020
Rate above prime	0.50%
Interest rate	5.00%	5.25%
Credit facility maximum borrowing capacity	$ 2,000,000	$ 50,000,000
Credit facility current borrowing capacity		2,000,000
Credit facility amount outstanding		0
Loans And Letters Of Credit [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 50,000,000
Maximum [Member]
Debt Instrument [Line Items]
Funded debt to EBITDA		3.5
Minimum [Member]
Debt Instrument [Line Items]
Current ratio		1
Interest coverage		3.0