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Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Income Tax Disclosure [Abstract] |  |
Alternative minimum tax and other credit carryforwards | $ 683.3 | $ 477.9
Accrued and unfunded compensation and employee benefits | 143.2 | 219.0
Amortizable intangible assets | 45.8 | 38.8
Compensation expense related to stock options | 13.4 | 16.9
Accrued liabilities | 28.9 | 31.7
Accrued pension liability | 14.8 | 22.9
Investments | 1.1 | 7.7
Net operating loss carryforwards | 30.5 | 20.4
Capital loss carryforwards | 12.9 | 16.0
Deferred rent liability | 4.8 | 8.1
Other | 5.5 | 3.9
Total deferred tax assets | 984.2 | 863.3
Valuation allowance for deferred tax assets | (79.1) | (66.8)
Deferred tax assets | 905.1 | 796.5
Nondeductible amortizable intangible assets | 273.8 | 310.2
Investment-related partnerships | 17.6 | 34.5
Depreciable fixed assets | 26.5 | 18.2
Hedging instruments | 3.8 | 4.1
Other prepaid items | 10.3 | 4.1
Total deferred tax liabilities | 332.0 | 371.1
Net deferred tax assets | $ 573.1 | $ 425.4

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