Investment Securities (Pledged Securities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Held-To-Maturity Securities for Federal Home Loan Bank and Federal Reserve Bank	$ 26.7
Carrying value of available-for-sale securities pledged to secure public and trust department deposits and repurchase agreements	330.0	260.9
Carrying value of held-to-maturity securities pledged to secure public and trust department deposits and repurchase agreements	18.4	45.3
Carrying value of available-for-sale securities pledged to secure additional borrowing capacity at FHLB and FRB	$ 23.5	$ 50.4