Derivative Instruments (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) item	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 51,100		$ 34,700
Interest payment period	7 years
Fixed interest rate	6.15%
Rate received by Southwest	3.87%
Pretax loss related to derivative contracts	$ 500		700
Gain or loss due to changes in the fair value of the derivative hedging instrument	100	$ 100
Net cash flows as a result of the interest rate swap agreement	100	$ 100
The fair value of cash and securities posted as collateral	600		700
Interest Rate Swap on Trust Preferred Securities [Member]
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 25,000
Variable interest rate	2.85%
Customer Risk Management Swaps [Member]
Derivative [Line Items]
Number of customer interest rate swap agreements | item	15
Interest rate swap agreement Notional amount	$ 160,100
Non-Hedge Derivative Asset [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount	160,125		123,953
Non-hedge derivative asset, fair value	2,043		1,235
Non-Hedge Derivative Liability [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	160,125		123,953
Non-hedge derivative liability, fair value	$ 2,043		$ 1,235