Parent Company Debt Schedule (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term Debt	$ 2,755.4	$ 2,570.2
Debt, Long-term and Short-term, Combined Amount	3,211.2	2,882.5
Notes Due 2042 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Long-term Debt	248.6	0
Notes Payable Due 2032 [Member]
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 [Member]
Debt Instrument [Line Items]
Long-term Debt	165.8	165.8
Notes 7 Point 25 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Notes Payable Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Long-term Debt	399.6	399.6
Notes Payable Due 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.125%
Long-term Debt	350.0	350.0
Notes Payable Due 2018 [Member]
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 [Member]
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Junior Subordinated Debt Securities Due 2038 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.405%
Long-term Debt	226.5	226.5
Parent Company [Member]
Debt Instrument [Line Items]
Long-term Debt	1,896.2	1,641.0
Debt, Long-term and Short-term, Combined Amount	1,902.8	1,643.5
Parent Company [Member] | Notes Due 2042 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Long-term Debt	248.6	0
Parent Company [Member] | Notes Payable Due 2032 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.375%
Long-term Debt	39.5	39.5
Parent Company [Member] | Notes at 6 Point 75 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	165.8	165.8
Parent Company [Member] | Notes 7 Point 25 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Parent Company [Member] | Notes Payable Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Long-term Debt	399.6	399.6
Parent Company [Member] | Notes Payable Due 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.125%
Long-term Debt	350.0	350.0
Parent Company [Member] | Notes Payable Due 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Long-term Debt	200.0	200.0
Parent Company [Member] | Medium-term Notes Due 2023 to 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Parent Company [Member] | Junior Subordinated Debt Securities Due 2038 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.405%
Long-term Debt	226.5	226.5
Parent Company [Member] | Fair Value Hedge Adjustment, LT Debt [Member]
Debt Instrument [Line Items]
Long-term Debt	15.4	8.8
Securities Pledged as Collateral [Member]
Debt Instrument [Line Items]
Short-term Debt	455.8	312.3
Securities Pledged as Collateral [Member] | Parent Company [Member]
Debt Instrument [Line Items]
Short-term Debt	$ 6.6	$ 2.5
Minimum [Member] | Notes Payable Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Minimum [Member] | Medium-term Notes Due 2023 to 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Minimum [Member] | Parent Company [Member] | Notes Payable Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Minimum [Member] | Parent Company [Member] | Medium-term Notes Due 2023 to 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Maximum [Member] | Notes Payable Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.25%
Maximum [Member] | Medium-term Notes Due 2023 to 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.20%
Maximum [Member] | Parent Company [Member] | Notes Payable Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.25%
Maximum [Member] | Parent Company [Member] | Medium-term Notes Due 2023 to 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.20%