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Debt (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
instrument
Dec. 31, 2016
USD ($)
instrument
Debt Instrument [Line Items]    
Fair Value $ 3,643.6 $ 3,315.8
Debt, Long-term and Short-term, Combined Amount [1] 3,306.3 3,148.2
3.75% Senior Notes due 2021    
Debt Instrument [Line Items]    
Carrying Value 498.8 498.4
Fair Value 520.7 528.8
2.45% Senior Notes Due 2027    
Debt Instrument [Line Items]    
Carrying Value 496.1 495.8
Fair Value 477.9 464.6
6 5/8% Senior Notes due 2029    
Debt Instrument [Line Items]    
Carrying Value 296.1 295.9
Fair Value 382.3 380.1
6.25% Senior Notes due 2032    
Debt Instrument [Line Items]    
Carrying Value 395.3 395.2
Fair Value 516.9 499.0
4.35% Senior Notes due 2044    
Debt Instrument [Line Items]    
Carrying Value 346.5 346.4
Fair Value 388.7 362.3
3.70% Senior Notes due 2045    
Debt Instrument [Line Items]    
Carrying Value 395.2 395.1
Fair Value 402.9 372.5
4.125% Senior Notes due 2047    
Debt Instrument [Line Items]    
Carrying Value 841.2 0.0
Fair Value 917.1 0.0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067    
Debt Instrument [Line Items]    
Carrying Value 0.0 594.1
Fair Value 0.0 581.2
Other Debt Instruments [Member]    
Debt Instrument [Line Items]    
Debt, Long-term and Short-term, Combined Amount $ 37.1 $ 127.3
Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Number of Debt Instruments Acquired | instrument 2 2
Debt, Long-term and Short-term, Combined Amount $ 37.1 $ 62.1
Trust Preferred Debt Securities [Member] | Other Debt Instruments [Member]    
Debt Instrument [Line Items]    
Carrying Value $ 0.0 $ 41.2
Debt Instrument, Number of Debt Instruments Acquired | instrument 0 2
Senior Notes [Member] | Other Debt Instruments [Member]    
Debt Instrument [Line Items]    
Carrying Value $ 0.0 $ 24.0
Debt Instrument, Number of Debt Instruments Acquired | instrument 0 4
[1] Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.