Derivative Instruments (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 26,100
Interest payment period	7 years
Fixed interest rate	6.15%
Rate received by Southwest	3.53%
Pretax loss related to derivative contracts	$ 900		$ 1,400
Gain or loss due to changes in the fair value of the derivative hedging instrument	(300)	$ 100
Net cash flows as a result of the interest rate swap agreement	500	$ 600
Fair Value Net of Cash Collateral	6,900
Interest Rate Swap on Trust Preferred Securities [Member]
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 25,000
Variable interest rate	2.85%
The fair value of cash and securities posted as collateral	$ 2,100		2,100
Customer Risk Management Swaps [Member]
Derivative [Line Items]
Number of customer interest rate swap agreements | item	11
Interest rate swap agreement Notional amount	$ 102,300
Non-Hedge Derivative Asset [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount	102,256		101,629
Non-hedge derivative asset, fair value	4,123		1,793
Non-Hedge Derivative Liability [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	102,256		101,629
Non-hedge derivative liability, fair value	$ 4,123		$ 1,793