Deferred Tax Liabilities and Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Tax Liabilities:
Lease Financing	$ 388.6	$ 392.0	$ 409.1
Software Development	316.1	299.0	277.8
Accumulated Depreciation	24.1	22.0	19.7
Compensation and Benefits	63.5	112.2	29.7
State Taxes, net	62.3ntrs_DeferredTaxLiabilitiesStateTaxes	63.3ntrs_DeferredTaxLiabilitiesStateTaxes	54.7ntrs_DeferredTaxLiabilitiesStateTaxes
Other Liabilities	157.5us-gaap_DeferredTaxLiabilitiesOther	104.0us-gaap_DeferredTaxLiabilitiesOther	170.9us-gaap_DeferredTaxLiabilitiesOther
Gross Deferred Tax Liabilities	1,012.1us-gaap_DeferredIncomeTaxLiabilities	992.5us-gaap_DeferredIncomeTaxLiabilities	961.9us-gaap_DeferredIncomeTaxLiabilities
Deferred Tax Assets:
Allowance for Credit Losses	103.5	107.8	114.7
Other Assets	126.7us-gaap_DeferredTaxAssetsOther	85.0us-gaap_DeferredTaxAssetsOther	118.4us-gaap_DeferredTaxAssetsOther
Gross Deferred Tax Assets	230.2us-gaap_DeferredTaxAssetsGross	192.8us-gaap_DeferredTaxAssetsGross	233.1us-gaap_DeferredTaxAssetsGross
Valuation Reserve	(3.9)	(3.9)	(3.9)
Deferred Tax Assets, net of Valuation Reserve	226.3us-gaap_DeferredTaxAssetsNet	188.9us-gaap_DeferredTaxAssetsNet	229.2us-gaap_DeferredTaxAssetsNet
Net Deferred Tax Liabilities	$ 785.8us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 803.6us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 732.7us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred Tax Liabilities, Compensation and Benefits

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Amount of deferred tax liability attributable to deductible temporary differences from state taxes.

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Amount of deferred tax liability attributable to taxable temporary differences.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the provision for loan losses.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences from capitalized software.

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Amount of deferred tax liability attributable to taxable temporary differences from leasing arrangements.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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