Allowance for Credit Losses - Recorded Investments in Loans and Leases (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases
Specifically Evaluated for Impairment	$ 92.2	$ 116.2
Evaluated for Inherent Impairment	31,317.4	32,373.8
Total Loans and Leases	31,409.6	32,490.0
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	5.0	7.2
Evaluated for Inherent Impairment	99.5	105.4
Allowance Assigned to Loans and Leases	104.5	112.6	$ 131.2
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	19.9	25.6	22.6
Total Allowance for Credit Losses	124.4	138.2	153.8	$ 192.0
Commercial
Loans and Leases
Specifically Evaluated for Impairment	10.4	15.0
Evaluated for Inherent Impairment	14,263.8	15,160.2
Total Loans and Leases	14,274.2	15,175.2
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	3.4	4.1
Evaluated for Inherent Impairment	54.7	53.5
Allowance Assigned to Loans and Leases	58.1	57.6	63.5
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	15.8	21.1	17.3
Total Allowance for Credit Losses	73.9	78.7	80.8	104.9
Personal
Loans and Leases
Specifically Evaluated for Impairment	81.8	101.2
Evaluated for Inherent Impairment	17,053.6	17,213.6
Total Loans and Leases	17,135.4	17,314.8
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	1.6	3.1
Evaluated for Inherent Impairment	44.8	51.9
Allowance Assigned to Loans and Leases	46.4	55.0	67.7
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	4.1	4.5	5.3
Total Allowance for Credit Losses	$ 50.5	$ 59.5	$ 73.0	$ 87.1

Allowance for Undrawn Exposure [Abstract]

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- Definition

This represents total allowance for credit losses established to cover probable credit losses related to loans and leases as well as off-balance sheet instruments including unfunded commitments and standby letters of credit. This differs from the US XBRL taxonomy tag in that the taxonomy tag does not include the allowance for off-balance sheet instruments in its scope.

+ References

No definition available.

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- Definition

Reflects the allowance for credit losses assigned to unfunded commitments and standby letters of credit as of the balance sheet date.

+ References

No definition available.

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- References

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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-Publisher FASB
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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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