Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Private equity investment Southwest's proportionate share of the net asset value	$ 2.4	$ 1.7
Impairment charges of mortgage loan servicing rights		0.5
Noncovered [Member]
Finite-Lived Intangible Assets [Line Items]
Noncovered impaired loans carrying amount	45.5	61.0
Noncovered impaired loans measured at fair value	32.3	46.9
Noncovered impaired loans life to date impairment	27.3	14.1
Provision for loan losses	0.1	9.8
Impairment charge of other real estate assets	1.5	4.0
Covered [Member]
Finite-Lived Intangible Assets [Line Items]
Impairment charge of other real estate assets	$ 0.7	$ 0.2