Parent Company Debt Schedule (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Long-term Debt	$ 2,570.2	$ 2,631.3
Securities Lending Agreements - See Note 3	312.3	0
Short-term Debt	312.3	225.1
Total Debt	2,882.5	2,856.4
Notes 7.625 Percent due 2011 callable at or above par [Member]
Debt Instrument [Line Items]
Short-term Debt	0	225.1
Notes 7.375 Percent due 2032 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	39.5	39.5
Notes at 6.75 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	165.8	165.8
Notes 7.25 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Notes 5.625 Percent due 2020 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	399.6	399.5
Notes 7.125 Percent due 2016 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	358.8	335.6
Notes 7.0 Percent due 2018 non-callable [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]
Debt Instrument [Line Items]
Long-term Debt	226.5	226.5
Parent Company [Member]
Debt Instrument [Line Items]
Long-term Debt	1,641.0	1,617.7
Securities Lending Agreements - See Note 3	2.5	0
Short-term Debt	2.5	225.1
Total Debt	1,643.5	1,842.8
Parent Company [Member] | Notes 7.625 Percent due 2011 callable at or above par [Member]
Debt Instrument [Line Items]
Short-term Debt	0	225.1
Parent Company [Member] | Notes 7.375 Percent due 2032 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	39.5	39.5
Parent Company [Member] | Notes at 6.75 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	165.8	165.8
Parent Company [Member] | Notes 7.25 Percent due 2028 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Parent Company [Member] | Notes 5.625 Percent due 2020 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	399.6	399.5
Parent Company [Member] | Notes 7.125 Percent due 2016 callable at or above par [Member]
Debt Instrument [Line Items]
Long-term Debt	358.8	335.6
Parent Company [Member] | Notes 7.0 Percent due 2018 non-callable [Member]
Debt Instrument [Line Items]
Long-term Debt	200.0	200.0
Parent Company [Member] | Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]
Debt Instrument [Line Items]
Long-term Debt	50.8	50.8
Parent Company [Member] | Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 226.5	$ 226.5