Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
ASSETS
Financial assets for which carrying value approximates fair value	$ 726.5	$ 846.8
Liabilities:
Deposits with no stated maturity and accrued interest	2,517.8	2,387.7
Securities sold under repurchase agreements and other short-term borrowing and accrued interest	34.4	34.6
Significant Other Observable Inputs (Level 2) [Member]
ASSETS
Held to maturity securities and accrued interest	6.6	10.8
Liabilities:
Time deposits and accrued interest	116.1	132.7
Other borrowings and accrued interest	14.6	14.2
Significant Unobservable Inputs (Level 3) [Member]
ASSETS
Loans and accrued interest	1,655.6	1,490.2
Carrying Amount [Member]
ASSETS
Financial assets for which carrying value approximates fair value	726.5	846.8
Held to maturity securities and accrued interest	6.2	10.2
FHLB Stock	3.5	4.8
Loans and accrued interest	1,637.3	1,453.2
Liabilities:
Deposits with no stated maturity and accrued interest	2,517.8	2,387.7
Time deposits and accrued interest	116.1	132.4
Securities sold under repurchase agreements and other short-term borrowing and accrued interest	34.4	34.6
Other borrowings and accrued interest	16.5	16.5
Total Fair Value [Member]
ASSETS
Financial assets for which carrying value approximates fair value	726.5	846.8
Held to maturity securities and accrued interest	6.6	10.8
FHLB Stock
Loans and accrued interest	1,655.6	1,490.2
Liabilities:
Deposits with no stated maturity and accrued interest	2,517.8	2,387.7
Time deposits and accrued interest	116.1	132.7
Securities sold under repurchase agreements and other short-term borrowing and accrued interest	34.4	34.6
Other borrowings and accrued interest	$ 14.6	$ 14.2