Parent Company Notes - Additional Details (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended										12 Months Ended						12 Months Ended			12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Notes 5 Point 750 Due 2042 Callable At or Above Par [Member]	Dec. 31, 2009 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2009 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2010 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member] Notes 7 Point 625 Percent due 2011 callable at or above par [Member]	Dec. 31, 2012 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member] Notes 7 Point 625 Percent due 2011 callable at or above par [Member]	Dec. 31, 2012 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 1998 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 2012 Notes @ 6.85% due 2015, callable at or above par	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2009 Parent Company [Member]	Dec. 31, 2012 Subsidiaries [Member] Notes @ 6.85% due 2015, callable at or above par	Dec. 31, 2011 Previous Accounting Guidance [Member]	Dec. 31, 2010 Previous Accounting Guidance [Member]	Dec. 31, 2011 Previous Accounting Guidance [Member] Parent Company [Member]	Dec. 31, 2011 Restatement Adjustment [Member]	Dec. 31, 2010 Restatement Adjustment [Member]	Dec. 31, 2011 Restatement Adjustment [Member] Parent Company [Member]	Dec. 31, 2010 Restatement Adjustment [Member] Parent Company [Member]	Dec. 31, 2012 Derivative Financial Instruments Receive Fixed Pay Variable [Member] Swap [Member]	Dec. 31, 2011 Derivative Financial Instruments Receive Fixed Pay Variable [Member] Swap [Member]
Impact of Restatement on Opening Retained Earnings, Net of Tax																							$ 455.1
Investments in Subsidiaries																				10,079.1	9,296.8						(407.3)
Accumulated Other Comprehensive Income (Loss), Net of Tax																				628.0	461.8						12.9
Retained Earnings (Accumulated Deficit)	7,371.6				6,611.0				7,371.6	6,611.0										7,371.6	6,611.0				7,031.2			(420.2)		(420.2)
Net Income	233.9	230.2	216.4	213.9	(369.0)	202.0	227.6	223.6	894.4	284.2	878.7									894.4	284.2	878.7			235.4	886.1		48.8	(7.4)	48.8	(7.4)
Long-term Debt, Maturities, Repayments of Principal in Year Four	350.0								350.0											350.0
Long-term Debt, Maturities, Repayments of Principal after Year Five	2,095.1								2,095.1											1,532.6
Proceeds from Unsecured Notes Payable												250.0	400.0
Debt Instrument, Decrease, Repayments														10.0
Notional Amount of Fair Value Hedge Instruments																																600.0	350.0
Junior Subordinated Notes																		300.0
Liquidation value per capital security																	$ 1,000
Debt Instrument, Interest Rate, Stated Percentage														7.08%		7.625%		7.405%	6.85%
Repayments of Medium-term Notes															225.1
Interest Paid									139.6	145.4	140.7									109.0	115.6	110.3
Guarantor Obligations, Liquidation Proceeds, Monetary Amount																			400.0
Long-term Debt	2,755.4				2,570.2				2,755.4	2,570.2										1,896.2	1,641.0			296.9
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries																				$ 670.8	$ 800.0	$ 576.0