Fair Value Disclosures (Change In Balance Sheet Carrying Value Associated With Level 3 Financial Assets Carried At Fair Value) (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, net	$ 57.5	$ 30.0
Total (losses) gains (realized/unrealized) included in earnings, net	(6.6)	6.0
Net unrealized gains (losses) included in other comprehensive income	(1.5)	(1.6)
Purchases, net	228.2	42.9
Sales, net	(35.5)	(10.5)
Transfers into Level 3, net	28.3	21.1
Transfers out of Level 3, net	(178.2)	(30.4)
Balance, end of period, net	92.2	57.5
Other liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, liabilities	(54.1)	(38.2)
Total (losses) gains (realized/unrealized) included in earnings, liabilities	0.7	8.6
Net unrealized (losses) gains included in other comprehensive income, liabilities	0.0	0.0
Purchases, liabilities	(3.4)	(24.5)
Sales, liabilities	0.3	0.0
Transfers into Level 3, liabilities	0.0	0.0
Transfers out of Level 3, liabilities	0.0	0.0
Balance, end of period, liabilities	(56.5)	(54.1)
States, municipalities and political subdivisions | Fixed maturity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	0.0	0.0
Total (losses) gains (realized/unrealized) included in earnings, assets		0.0
Net unrealized (losses) gains included in other comprehensive income, assets		0.0
Purchases, assets		3.6
Sales, assets		(3.6)
Transfers into Level 3, assets		0.0
Transfers out of Level 3, assets		0.0
Balance, end of period, assets		0.0
Asset-backed | Fixed maturity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	0.0
Total (losses) gains (realized/unrealized) included in earnings, assets	3.3
Net unrealized (losses) gains included in other comprehensive income, assets	(2.2)
Purchases, assets	165.6
Sales, assets	(20.0)
Transfers into Level 3, assets	2.6
Transfers out of Level 3, assets	(109.9)
Balance, end of period, assets	39.4	0.0
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	27.8	0.2
Total (losses) gains (realized/unrealized) included in earnings, assets	(4.9)	(2.0)
Net unrealized (losses) gains included in other comprehensive income, assets	0.3	(1.0)
Purchases, assets	5.4	30.8
Sales, assets	0.0	(0.2)
Transfers into Level 3, assets	0.0	0.0
Transfers out of Level 3, assets	0.0	0.0
Balance, end of period, assets	28.6	27.8
U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	44.5	34.5
Total (losses) gains (realized/unrealized) included in earnings, assets	0.6	0.4
Net unrealized (losses) gains included in other comprehensive income, assets	0.8	(0.1)
Purchases, assets	15.4	28.9
Sales, assets	(13.4)	(5.1)
Transfers into Level 3, assets	9.2	16.3
Transfers out of Level 3, assets	(36.0)	(30.4)
Balance, end of period, assets	21.1	44.5
Foreign corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	33.3	28.6
Total (losses) gains (realized/unrealized) included in earnings, assets	(0.1)	0.1
Net unrealized (losses) gains included in other comprehensive income, assets	(0.3)	(0.4)
Purchases, assets	25.5	1.7
Sales, assets	(2.3)	(1.5)
Transfers into Level 3, assets	16.5	4.8
Transfers out of Level 3, assets	(27.3)	0.0
Balance, end of period, assets	45.3	33.3
Non-redeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	2.2	2.3
Total (losses) gains (realized/unrealized) included in earnings, assets	0.0	0.0
Net unrealized (losses) gains included in other comprehensive income, assets	0.0	(0.1)
Purchases, assets	0.0	0.0
Sales, assets	0.0	0.0
Transfers into Level 3, assets	0.0	0.0
Transfers out of Level 3, assets	0.0	0.0
Balance, end of period, assets	2.2	2.2
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	3.5	2.2
Total (losses) gains (realized/unrealized) included in earnings, assets	(6.0)	(1.0)
Net unrealized (losses) gains included in other comprehensive income, assets	(0.1)	0.0
Purchases, assets	17.7	2.4
Sales, assets	(0.1)	(0.1)
Transfers into Level 3, assets	0.0	0.0
Transfers out of Level 3, assets	(5.0)	0.0
Balance, end of period, assets	10.0	3.5
Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period, assets	0.3	0.4
Total (losses) gains (realized/unrealized) included in earnings, assets	(0.2)	(0.1)
Net unrealized (losses) gains included in other comprehensive income, assets	0.0	0.0
Purchases, assets	2.0	0.0
Sales, assets	0.0	0.0
Transfers into Level 3, assets	0.0	0.0
Transfers out of Level 3, assets	0.0	0.0
Balance, end of period, assets	$ 2.1	$ 0.3