Derivative Instruments (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2016 USD ($) item	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Interest payment period			7 years
Fixed interest rate			6.15%
Rate received by Southwest			3.48%
Pretax loss related to derivative contracts			$ 1,200		$ 1,400
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	$ 300	$ 300		$ 400
Fair Value Net of Cash Collateral			7,400
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		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			$ 98,300
Non-Hedge Derivative Asset [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount			98,318		101,629
Non-hedge derivative asset, fair value			5,163		1,793
Non-Hedge Derivative Liability [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount			98,318		101,629
Non-hedge derivative liability, fair value			$ 5,163		$ 1,793