Parent Company Debt Schedule (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term Debt	$ 2,612.0	$ 2,755.4
Debt, Long-term and Short-term, Combined Amount	2,688.5	3,211.2
Notes Due 2042
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Long-term Debt	248.6	248.6
Notes Payable Due 2032
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.375%
Long-term Debt	39.5	39.5
Notes at 6 Point 75 Percent due 2028 callable at or above par
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Long-term Debt	165.8	165.8
Notes 7 Point 25 Percent due 2028 callable at or above par
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.25%
Long-term Debt	200.0	200.0
Notes Payable Due 2020
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Long-term Debt	399.7	399.6
Notes Payable Due 2016
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.125%
Long-term Debt	350.0	350.0
Notes Payable Due 2018
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Long-term Debt	200.0	200.0
Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Junior Subordinated Debt Securities Due 2038
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.405%
Long-term Debt	226.5	226.5
Parent Company
Debt Instrument [Line Items]
Long-term Debt	1,875.2	1,896.2
Debt, Long-term and Short-term, Combined Amount	1,875.5	1,902.8
Parent Company | Notes Due 2042
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Long-term Debt	248.6	248.6
Parent Company | Notes Payable Due 2032
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.375%
Long-term Debt	39.5	39.5
Parent Company | Notes at 6 Point 75 Percent due 2028 callable at or above par
Debt Instrument [Line Items]
Long-term Debt	165.8	165.8
Parent Company | Notes 7 Point 25 Percent due 2028 callable at or above par
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Parent Company | Notes Payable Due 2020
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Long-term Debt	399.7	399.6
Parent Company | Notes Payable Due 2016
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.125%
Long-term Debt	350.0	350.0
Parent Company | Notes Payable Due 2018
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Long-term Debt	200.0	200.0
Parent Company | Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Parent Company | Junior Subordinated Debt Securities Due 2038
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.405%
Long-term Debt	226.5	226.5
Parent Company | Fair Value Hedge Adjustment, LT Debt
Debt Instrument [Line Items]
Long-term Debt	(5.7)	15.4
Securities Pledged as Collateral
Debt Instrument [Line Items]
Short-term Debt	76.5	455.8
Securities Pledged as Collateral | Parent Company
Debt Instrument [Line Items]
Short-term Debt	$ 0.3	$ 6.6
Minimum | Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Minimum | Parent Company | Notes Payable Due 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Minimum | Parent Company | Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Maximum | Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.20%
Maximum | Parent Company | Notes Payable Due 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.25%
Maximum | Parent Company | Medium-term Notes Due 2023 to 2028
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.20%