Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 4.6	$ 37.9	$ 29.7	$ 13.5	$ 65.4	$ 4.7	$ 9.3	$ 35.1	$ 85.7	$ 114.5	$ 217.7
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Parent									(151.9)	259.7	(324.9)
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Parent									(25.4)	(25.1)	(37.9)
Unrealized Holding Gains (Losses)									(177.3)	234.6	(362.8)
Unrecognized Postretirement Benefit Costs Arising During the Year									3.0	(106.4)	61.6
Net Unrecognized Postretirement Benefit Costs									26.1	(98.5)	86.6
Foreign Currency Translation Adjustments									(1.4)	(1.0)	0.2
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustments									(1.4)	(1.0)	0.2
Other Comprehensive Income (Loss) Before Income Taxes									(152.6)	135.1	(276.0)
Income Tax Benefit (Expense):
Securities Held by Parent									53.9	(91.6)	115.9
Securities Held by Parent									8.9	8.8	13.2
Unrealized Holding Gains and Losses									62.8	(82.8)	129.1
Unrecognized Postretirement Benefit Costs Arising During the Year									(1.1)	37.5	(21.6)
Net Unrecognized Postretirement Benefit Costs									(9.1)	34.7	(30.4)
Foreign Currency Translation Adjustments									0.5	0.4	(0.1)
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustment									0.5	0.4	(0.1)
Other Comprehensive Income Tax Benefit (Expense)									54.2	(47.7)	98.6
Other Comprehensive Income (Loss)									(98.4)	87.4	(177.4)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income									85.7	114.5	217.7
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Subsidiaries									(150.4)	261.7	(324.9)
Securities Held by Parent									(1.5)	(2.0)	0.0
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Subsidiaries									(26.9)	(26.8)	(37.9)
Securities Held by Parent									1.5	1.7	0.0
Unrealized Holding Gains (Losses)									(177.3)	234.6	(362.8)
Unrecognized Postretirement Benefit Costs Arising During the Year									3.0	(106.4)	61.6
Amortization of Unrecognized Postretirement Benefit Costs									23.1	7.9	25.0
Net Unrecognized Postretirement Benefit Costs									26.1	(98.5)	86.6
Foreign Currency Translation Adjustments									(1.4)	(1.0)	0.2
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustments									(1.4)	(1.0)	0.2
Other Comprehensive Income (Loss) Before Income Taxes									(152.6)	135.1	(276.0)
Income Tax Benefit (Expense):
Securities Held by Subsidiaries									53.4	(92.3)	115.9
Securities Held by Parent									0.5	0.7	0.0
Securities Held by Subsidiaries									9.4	9.4	13.2
Securities Held by Parent									(0.5)	(0.6)	0.0
Unrealized Holding Gains and Losses									62.8	(82.8)	129.1
Unrecognized Postretirement Benefit Costs Arising During the Year									(1.1)	37.5	(21.6)
Amortization of Unrecognized Postretirement Benefit Costs									(8.0)	(2.8)	(8.8)
Net Unrecognized Postretirement Benefit Costs									(9.1)	34.7	(30.4)
Foreign Currency Translation Adjustments									0.5	0.4	(0.1)
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustment									0.5	0.4	(0.1)
Other Comprehensive Income Tax Benefit (Expense)									54.2	(47.7)	98.6
Other Comprehensive Income (Loss)									(98.4)	87.4	(177.4)
Total Comprehensive Income (Loss)									$ (12.7)	$ 201.9	$ 40.3