Investments (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Investments
Percent of Total (as a percent)	100.00%	100.00%
Amortized Cost, total investment portfolio	$ 10,666.5	$ 10,320.7
Gross Unrealized Gains, total investment portfolio	352.1	260.1
Gross Unrealized Losses, total investment portfolio	(84.0)	(133.9)
Estimated Fair Value, total investment portfolio	10,934.6	10,446.9
AOCI Gain (Loss) on Securities with OTTI	0.9	(11.4)
Percentage of government agency obligations (as a percent)	63.00%	64.00%
Unrealized gains (losses) included in AOCI for recognized OTTI, net of tax	2.9	5.6
Unrealized gains (losses) included in AOCI for not recognized OTTI, net of tax	199.3	114.6
Fixed maturity securities
Investments
Percent of Total (as a percent)	90.00%	90.00%
Amortized Cost, total fixed maturity securities	9,596.1	9,289.4
Gross Unrealized Gains, total fixed maturity securities	352.1	259.8
Gross Unrealized Losses, total fixed maturity securities	(84.0)	(133.9)
Estimated Fair Value, total fixed maturity securities	9,864.2	9,415.3
AOCI Gain (Loss) on Securities with OTTI	0.9	(11.4)
U.S. government and agencies
Investments
Percent of Total (as a percent)	9.00%	10.00%
Amortized Cost, total fixed maturity securities	926.1	1,000.3
Gross Unrealized Gains, total fixed maturity securities	49.4	48.3
Gross Unrealized Losses, total fixed maturity securities	(0.2)	(0.4)
Estimated Fair Value, total fixed maturity securities	975.3	1,048.2
Obligations of state and political subdivisions
Investments
Percent of Total (as a percent)	47.00%	48.00%
Amortized Cost, total fixed maturity securities	5,038.7	4,922.0
Gross Unrealized Gains, total fixed maturity securities	162.3	99.9
Gross Unrealized Losses, total fixed maturity securities	(16.8)	(62.0)
Estimated Fair Value, total fixed maturity securities	5,184.2	4,959.9
AOCI Gain (Loss) on Securities with OTTI	1.8	(1.4)
Corporate securities
Investments
Percent of Total (as a percent)	10.00%	9.00%
Amortized Cost, total fixed maturity securities	1,045.7	980.1
Gross Unrealized Gains, total fixed maturity securities	28.7	25.2
Gross Unrealized Losses, total fixed maturity securities	(8.9)	(12.8)
Estimated Fair Value, total fixed maturity securities	1,065.5	992.5
AOCI Gain (Loss) on Securities with OTTI	(0.2)	0.2
RMBS
Investments
Percent of Total (as a percent)	11.00%	11.00%
Amortized Cost, total fixed maturity securities	1,208.8	1,173.6
Gross Unrealized Gains, total fixed maturity securities	59.9	56.4
Gross Unrealized Losses, total fixed maturity securities	(51.2)	(45.9)
Estimated Fair Value, total fixed maturity securities	1,217.5	1,184.1
AOCI Gain (Loss) on Securities with OTTI	(23.0)	(8.6)
CMBS
Investments
Percent of Total (as a percent)	5.00%	4.00%
Amortized Cost, total fixed maturity securities	498.8	365.7
Gross Unrealized Gains, total fixed maturity securities	17.7	14.8
Gross Unrealized Losses, total fixed maturity securities	(0.4)	(1.4)
Estimated Fair Value, total fixed maturity securities	516.1	379.1
AOCI Gain (Loss) on Securities with OTTI	2.7	2.5
Asset-backed securities
Investments
Percent of Total (as a percent)	5.00%	5.00%
Amortized Cost, total fixed maturity securities	540.4	498.2
Gross Unrealized Gains, total fixed maturity securities	26.0	9.9
Gross Unrealized Losses, total fixed maturity securities	(5.5)	(5.2)
Estimated Fair Value, total fixed maturity securities	560.9	502.9
AOCI Gain (Loss) on Securities with OTTI	19.6	(4.1)
Foreign government securities
Investments
Percent of Total (as a percent)	3.00%	3.00%
Amortized Cost, total fixed maturity securities	337.6	349.5
Gross Unrealized Gains, total fixed maturity securities	8.1	5.3
Gross Unrealized Losses, total fixed maturity securities	(1.0)	(6.2)
Estimated Fair Value, total fixed maturity securities	344.7	348.6
Short-term investments.
Investments
Percent of Total (as a percent)	10.00%	10.00%
Amortized Cost, total investment portfolio	1,070.4	1,031.3
Gross Unrealized Gains, total investment portfolio		0.3
Estimated Fair Value, total investment portfolio	$ 1,070.4	$ 1,031.6

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