Consolidated Statements of Shareholders' Equity (USD $) In Millions, unless otherwise specified	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments and Derivative Instruments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Total Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2007			$ 0.6	$ 1,822.6	$ 5.5	$ (25.9)		$ 946.9	$ (450.7)
Cumulative effect of change in accounting principle, net of tax
Balance at beginning of year, as adjusted					5.5			946.9
Amounts arising in the period	3.1
Amounts arising in the period	(123.8)					(123.8)
Shares purchased at cost									(58.5)
Settlement of accelerated share repurchases
Employee and director stock-based awards and other				(18.8)
Amortization during the period:
Amount recognized as net periodic benefit cost	(3.6)					(3.6)
Benefit (provision) for deferred federal income taxes	44.6					44.6
Total pension and postretirement benefits	(82.8)					(82.8)
Net shares reissued at cost under employee stock-based compensation plans									27.0
Net income	20.6							20.6
Net appreciation (depreciation) during the period:
Other comprehensive income (loss)	(364.4)
Net appreciation (depreciation) on available-for-sale securities and derivative instruments					(284.3)
Benefit for deferred federal income taxes					2.7
Other Comprehensive income (loss)	(281.6)				(281.6)
Dividends to shareholders								(23.0)
Treasury stock issued for less than cost and other								(9.7)
Recognition of share-based compensation								15.0
Balance at Dec. 31, 2008	1,887.2		0.6	1,803.8	(276.1)	(108.7)	(384.8)	949.8	(482.2)
Cumulative effect of change in accounting principle, net of tax					(33.3)			33.3
Balance at beginning of year, as adjusted					(309.4)	(108.7)		983.1
Amounts arising in the period	4.5
Amounts arising in the period	26.0					26.0
Shares purchased at cost									(148.1)
Employee and director stock-based awards and other				4.7
Amortization during the period:
Amount recognized as net periodic benefit cost	19.8					19.8
Benefit (provision) for deferred federal income taxes	(16.0)					(16.0)
Total pension and postretirement benefits	29.8					29.8
Net shares reissued at cost under employee stock-based compensation plans									9.9
Net income	197.2							197.2
Net appreciation (depreciation) during the period:
Other comprehensive income (loss)	446.9
Net appreciation (depreciation) on available-for-sale securities and derivative instruments					415.8
Benefit for deferred federal income taxes					1.3
Other Comprehensive income (loss)	417.1				417.1
Dividends to shareholders								(37.5)
Treasury stock issued for less than cost and other								(5.3)
Recognition of share-based compensation								3.6
Balance at Dec. 31, 2009	2,358.6		0.6	1,808.5	107.7	(78.9)	28.8	1,141.1	(620.4)
Cumulative effect of change in accounting principle, net of tax
Balance at beginning of year, as adjusted					107.7			1,141.1
Amounts arising in the period						(13.9)
Amounts arising in the period	(13.9)
Shares purchased at cost									(126.0)
Settlement of accelerated share repurchases				(8.7)
Employee and director stock-based awards and other				(3.3)
Amortization during the period:
Amount recognized as net periodic benefit cost	9.8					9.8
Benefit (provision) for deferred federal income taxes	1.4					1.4
Total pension and postretirement benefits	(2.7)					(2.7)
Net shares reissued at cost under employee stock-based compensation plans									26.3
Net income	154.8							154.8
Net appreciation (depreciation) during the period:
Other comprehensive income (loss)	107.9
Net appreciation (depreciation) on available-for-sale securities and derivative instruments					104.7
Benefit for deferred federal income taxes					5.9
Other Comprehensive income (loss)	110.6				110.6
Dividends to shareholders								(47.2)
Treasury stock issued for less than cost and other								(9.7)
Recognition of share-based compensation								7.8
Balance at Dec. 31, 2010	$ 2,460.5		$ 0.6	$ 1,796.5	$ 218.3	$ (81.6)	$ 136.7	$ 1,246.8	$ (720.1)