Derivative Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) agreement	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 34,700
The fair value of cash and securities posted as collateral	700	$ 1,400
Interest Rate Swap on Trust Preferred Securities [Member]
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 25,000
Interest payment period	7 years
Fixed interest rate	6.15%
Variable interest rate	2.85%
Rate received by Southwest	3.73%
Pretax loss related to derivative contracts	$ 700	1,400
Gain or loss due to changes in the fair value of the derivative hedging instrument	400	100
Net cash flows as a result of the interest rate swap agreement	$ 700	800	$ 800
Customer Risk Management Swaps [Member]
Derivative [Line Items]
Number of customer interest rate swap agreements | agreement	9
Derivative Asset, Notional Amount	$ 123,953	101,629
Derivative Liability, Notional Amount	123,953	101,629
Non-hedge derivative asset, fair value	1,235	1,793
Non-hedge derivative liability, fair value	$ 1,235	$ 1,793