Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative Instruments [Abstract]
Interest rate swap agreement Notional amount	$ 25.0
Interest payment period	7 years
Fixed interest rate	6.15%
Variable interest rate	2.85%
Rate received by Southwest	3.12%
Pretax loss related to derivative contracts	2.2		3.2
Gain or loss due to changes in the fair value of the derivative hedging instrument	0.6	0.3
Net cash flows as a result of the interest rate swap agreement	0.6	0.5
The fair value of cash and securities posted as collateral	$ 4.1		$ 4.2