Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Private equity investment Southwest's proportionate share of the net asset value	$ 1.7	$ 1.2
Impairment charges of mortgage loan servicing rights	0.5	0.3
Noncovered [Member]
Finite-Lived Intangible Assets [Line Items]
Noncovered impaired loans carrying amount	61.0	19.7
Noncovered impaired loans measured at fair value	46.9	11.5
Noncovered impaired loans life to date impairment	14.1	8.1
Provision for loan losses	9.8	7.4
Impairment charge of other real estate assets	4.0	3.1
Covered [Member]
Finite-Lived Intangible Assets [Line Items]
Impairment charge of other real estate assets	$ 0.2