Fair Value (Fair Value Of Assets On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Federal Home Loan Bank common stock	$ 9.7		$ 9.4
Fixed maturities	6,470.6	6,284.7
Cash and cash equivalents	628.3	820.4	218.8	290.4
Mortgage loans	4.8
Other investments	149.4	135.1
Total investment assets at fair value	7,587.2	6,656.8
Debt	997.6
Total liabilities at fair value	997.6
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,470.6	6,284.7
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	628.3
Total investment assets at fair value	1,027.8	324.7
Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	144.2	147.3
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans	4.8
Other investments	149.4	135.1
Total investment assets at fair value	6,462.8	6,233.2
Debt	997.6
Total liabilities at fair value	997.6
Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,251.2	6,061.9
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	96.6	98.9
Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	75.2	75.5
U.S. Treasury And Government Agencies [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	231.0	269.3
U.S. Treasury And Government Agencies [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	89.2	147.3
U.S. Treasury And Government Agencies [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	141.8	122.0
Foreign Governments [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	273.8	239.0
Foreign Governments [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	55.0
Foreign Governments [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	218.8	239.0
Municipal [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,005.0	1,028.0
Municipal [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	988.0	1,014.4
Municipal [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	17.0	13.6
Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,545.6	3,375.6
Corporate [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,516.9	3,351.8
Corporate [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	28.7	23.8
Residential Mortgage-Backed, U.S. Agency Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	653.0	663.3
Residential Mortgage-Backed, U.S. Agency Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	653.0	663.3
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	158.4	185.3
Residential Mortgage-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	157.7	180.1
Residential Mortgage-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.7	5.2
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	387.4	379.1
Commercial Mortgage-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	382.9	374.4
Commercial Mortgage-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4.5	4.7
Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	216.4	145.1
Asset-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	192.1	116.9
Asset-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	24.3	28.2
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	334.1	237.0
Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	255.3	177.4
Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	57.4	36.2
Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 21.4	$ 23.4