Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 65.4	$ 4.7	$ 9.3	$ 35.1	$ 55.2	$ 70.1	$ 34.0	$ 58.4	$ 114.5	$ 217.7	$ 103.4
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Parent									259.7	(324.9)	157.9
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Parent									(25.1)	(37.9)	(61.2)
Unrealized Holding Gains (Losses)									(234.6)	362.8	(96.7)
Unrecognized Postretirement Benefit Costs Arising During the Year									(106.4)	61.6	(29.4)
Net Unrecognized Postretirement Benefit Costs									(98.5)	86.6	(13.2)
Foreign Currency Translation Adjustments									(1.0)	0.2	1.6
Reclassification Adjustment for Amounts Included in Net Income									0	0	0
Foreign Currency Translation Adjustments									(1.0)	0.2	1.6
Other Comprehensive Income (Loss) Before Income Taxes									135.1	(276.0)	85.1
Income Tax Benefit (Expense):
Securities Held by Parent									91.6	(115.9)	55.9
Securities Held by Parent									(8.8)	(13.2)	(21.5)
Unrealized Holding Gains and Losses									(82.8)	129.1	(34.4)
Unrecognized Postretirement Benefit Costs Arising During the Year									37.5	(21.6)	10.3
Net Unrecognized Postretirement Benefit Costs									34.7	(30.4)	4.6
Foreign Currency Translation Adjustments									0.4	(0.1)	(0.6)
Reclassification Adjustment for Amounts Included in Net Income									0	0	0
Foreign Currency Translation Adjustment									0.4	(0.1)	(0.6)
Other Comprehensive Income Tax Benefit (Expense)									(47.7)	98.6	(30.4)
Other Comprehensive Income (Loss)									87.4	(177.4)	54.7
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income									114.5	217.7	103.4
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Subsidiaries									261.7	(324.9)	157.9
Securities Held by Parent									(2.0)	0	0
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Subsidiaries									(26.8)	(37.9)	(61.2)
Securities Held by Parent									1.7	0	0
Unrealized Holding Gains (Losses)									(234.6)	362.8	(96.7)
Unrecognized Postretirement Benefit Costs Arising During the Year									(106.4)	61.6	(29.4)
Amortization of Unrecognized Postretirement Benefit Costs									7.9	25.0	16.2
Net Unrecognized Postretirement Benefit Costs									(98.5)	86.6	(13.2)
Foreign Currency Translation Adjustments									(1.0)	0.2	1.6
Reclassification Adjustment for Amounts Included in Net Income									0	0	0
Foreign Currency Translation Adjustments									(1.0)	0.2	1.6
Other Comprehensive Income (Loss) Before Income Taxes									135.1	(276.0)	85.1
Income Tax Benefit (Expense):
Securities Held by Subsidiaries									(92.3)	115.9	(55.9)
Securities Held by Parent									(0.7)	0	0
Securities Held by Subsidiaries									9.4	13.2	21.5
Securities Held by Parent									0.6	0	0
Unrealized Holding Gains and Losses									(82.8)	129.1	(34.4)
Unrecognized Postretirement Benefit Costs Arising During the Year									37.5	(21.6)	10.3
Amortization of Unrecognized Postretirement Benefit Costs									(2.8)	(8.8)	(5.7)
Net Unrecognized Postretirement Benefit Costs									34.7	(30.4)	4.6
Foreign Currency Translation Adjustments									0.4	(0.1)	(0.6)
Reclassification Adjustment for Amounts Included in Net Income									0	0	0
Foreign Currency Translation Adjustment									0.4	(0.1)	(0.6)
Other Comprehensive Income Tax Benefit (Expense)									(47.7)	98.6	(30.4)
Other Comprehensive Income (Loss)									87.4	(177.4)	54.7
Total Comprehensive Income									$ 201.9	$ 40.3	$ 158.1