Derivative Instruments (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Interest payment period	7 years
Fixed interest rate	6.15%
Rate received by Southwest	3.47%
Pretax loss related to derivative contracts	$ 1,300		$ 1,400
Gain or loss due to changes in the fair value of the derivative hedging instrument	100	$ 50
Net cash flows as a result of the interest rate swap agreement	200	$ 200
Fair Value Net of Cash Collateral	8,000
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	9
Interest rate swap agreement Notional amount	$ 100,800
Non-Hedge Derivative Asset [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount	100,846		101,629
Non-hedge derivative asset, fair value	4,307		1,793
Non-Hedge Derivative Liability [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	100,846		101,629
Non-hedge derivative liability, fair value	$ 4,307		$ 1,793