Derivative Instruments (Details) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Interest rate swap agreement Notional amount	$ 51,600		$ 34,700
Interest payment period	7 years
Fixed interest rate	6.15%
Rate received by Southwest	4.01%
Pretax loss related to derivative contracts	$ 400		700
Gain or loss due to changes in the fair value of the derivative hedging instrument	(200)	$ (100)
Net cash flows as a result of the interest rate swap agreement	300	$ 300
The fair value of cash and securities posted as collateral	500		700
Fair Value Net of Cash Collateral	1,500
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	17
Interest rate swap agreement Notional amount	$ 180,600
Non-Hedge Derivative Asset [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount	180,631		123,953
Non-hedge derivative asset, fair value	2,698		1,235
Non-Hedge Derivative Liability [Member]
Derivative [Line Items]
Derivative Liability, Notional Amount	180,631		123,953
Non-hedge derivative liability, fair value	$ 2,698		$ 1,235