Parent Company Notes - Additional Details (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended							12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Notes 7.625 Percent due 2011 callable at or above par [Member]	Dec. 31, 2009 Notes @ 5.859 - Short term [Member]	Dec. 31, 2009 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2010 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2010 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2009 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2009 Notes at 6.75 Percent due 2028 callable at or above par [Member]	Dec. 31, 2011 Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]	Dec. 31, 1998 Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2009 Parent Company [Member]	Dec. 31, 2005 Subsidiaries [Member] Notes 6.85 Percent due 2015 callable at or above par[Member]	Dec. 31, 2011 Subsidiaries [Member] Notes 6.85 Percent due 2015 callable at or above par[Member]
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 350.0												$ 350.0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,915.1												1,282.6
Proceeds from Unsecured Notes Payable						350.0										400.0
Repayments of Medium-term Notes								10.0
Debt Instrument, Interest Rate, Stated Percentage				7.625%	5.859%		7.125%	7.08%	7.19%	6.75%		7.405%					6.85%
Debt Instrument, Decrease, Repayments									1.2	0.6
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	8.8	(14.4)
Junior Subordinated Notes												300.0
Liquidation value per capital security											$ 1,000
Repayments of Long-term Debt	84.4	78.3	59.8	225.1	132.2								0	10.0	1.8
Interest Paid	145.4	140.7	122.0										115.6	110.3	84.5
Long-term Debt	2,570.2	2,631.3											1,641.0	1,617.7			296.9
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries													$ 800.0	$ 576.0	$ 313.8