Debt (Detail) $ in Millions		1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
		Jan. 26, 2015 USD ($)	Apr. 25, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Carrying Value				$ 2,739.0	$ 2,208.0	$ 2,739.0	$ 2,208.0	$ 2,164.7
Fair Value				2,942.4	2,551.6	2,942.4	2,551.6	2,527.5
Debt, Long-term and Short-term, Combined Amount	[1]			2,739.0	2,208.0	2,739.0	2,208.0	2,164.7
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest				368.5	293.4	664.1	614.7
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value				498.0	497.7	498.0	497.7	497.8
Fair Value				533.0	532.8	533.0	532.8	535.6
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value				295.6	295.4	295.6	295.4	295.5
Fair Value				380.9	395.9	380.9	395.9	400.6
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value				394.9	394.7	394.9	394.7	394.8
Fair Value				492.1	514.7	492.1	514.7	527.9
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Carrying Value				346.3	346.3	346.3	346.3	346.3
Fair Value				346.5	355.0	346.5	355.0	378.9
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)			$ (1.6)
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Carrying Value				395.0	0.0	395.0	0.0	0.0
Fair Value				351.7	0.0	351.7	0.0	0.0
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)		$ (12.9)
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Extinguishment of Debt, Amount						0.0	0.0	44.3
Carrying Value				630.7	673.9	630.7	673.9	630.3
Fair Value				659.7	753.2	659.7	753.2	684.5
Gains (Losses) on Extinguishment of Debt						0.0	0.0	(4.8)
Other Debt Instruments [Member]
Debt Instrument [Line Items]
Carrying Value				178.5	0.0	178.5	0.0	0.0
Fair Value					$ 0.0		0.0	0.0
Debt, Long-term and Short-term, Combined Amount				$ 178.5		178.5
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						$ 0.0	$ 0.0
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)								$ 0.5
Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Number of debt instruments				2,000,000		2,000,000
Debt, Long-term and Short-term, Combined Amount				$ 99.5		$ 99.5
Trust Preferred Debt Securities [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Carrying Value				$ 41.3		$ 41.3
Number of debt instruments				2,000,000		2,000,000
Senior Notes [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Carrying Value				$ 24.0		$ 24.0
Number of debt instruments				4,000,000		4,000,000
Surplus Note [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Number of debt instruments				1,000,000		1,000,000
Debt, Long-term and Short-term, Combined Amount				$ 13.7		$ 13.7

[1] 1Consists of both short-term and long-term debt. See Note 4 - Debt.See notes to consolidated financial statements.