Investments - Amortized Cost and Estimated Fair Value of investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,341.7	$ 4,370.5
Gross Unrealized Gains	455.6	284.2
Gross Unrealized Losses	(19.7)	(79.7)
Fair Value	4,777.6	4,575.0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	315.2	351.1
Gross Unrealized Gains	32.3	22.8
Gross Unrealized Losses	(2.0)	(11.7)
Fair Value	345.5	362.2
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,352.5	1,327.4
Gross Unrealized Gains	126.4	53.8
Gross Unrealized Losses	(1.8)	(20.2)
Fair Value	1,477.1	1,361.0
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,599.3	2,636.4
Gross Unrealized Gains	294.3	205.0
Gross Unrealized Losses	(15.1)	(47.7)
Fair Value	2,878.5	2,793.7
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5.9	6.6
Gross Unrealized Gains	0.8	0.8
Gross Unrealized Losses	0	0
Fair Value	6.7	7.4
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64.9	44.2
Gross Unrealized Gains	0.3	0.5
Gross Unrealized Losses	(0.8)	0
Fair Value	64.4	44.7
Other Mortgage and Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3.9	4.8
Gross Unrealized Gains	1.5	1.3
Gross Unrealized Losses	0	(0.1)
Fair Value	5.4	6.0
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	3.9	4.6
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1.3	1.2
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 0.2	$ 0.2

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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