Fair Value (Fair Value of Assets on a Recurring Basis) (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Fixed maturities	$ 4,697.9	$ 4,797.9
Total investment assets at fair value	4,853.4	4,917.9
Derivative instruments	4.7
U.S. Treasury and Government Agencies [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities	94.6	124.0
U.S. Treasury and Government Agencies [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	119.8	137.2
U.S. Treasury and Government Agencies [Member] | Fixed Maturities [Member]
Fixed maturities	214.4	261.2
Corporate [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Corporate [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	2,380.3	2,392.2
Corporate [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	36.8	28.2
Corporate [Member] | Fixed Maturities [Member]
Fixed maturities	2,417.1	2,420.4
Residential Mortgage-Backed [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Residential Mortgage-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	100.5	132.9
Residential Mortgage-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	0.5	0.8
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fixed maturities	101.0	133.7
Residential Mortgage-Backed Securities, U.S. Agency Backed [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Residential Mortgage-Backed Securities, U.S. Agency Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	598.6	600.4
Residential Mortgage-Backed Securities, U.S. Agency Backed [Member] | Fixed Maturities [Member]
Fixed maturities	598.6	600.4
Commercial Mortgage-Backed [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Commercial Mortgage-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	349.0	361.1
Commercial Mortgage-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	5.1	5.5
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Fixed maturities	354.1	366.6
FHLBB Borrowings [Member]
Excluded FHLBB common stock	8.7	8.6
Municipal [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Municipal [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	953.0	938.1
Municipal [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	16.1	16.6
Municipal [Member] | Fixed Maturities [Member]
Fixed maturities	969.1	954.7
Asset-Backed [Member] | Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities
Asset-Backed [Member] | Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	30.1	47.4
Asset-Backed [Member] | Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	13.5	13.5
Asset-Backed [Member] | Fixed Maturities [Member]
Fixed maturities	43.6	60.9
Level 1 [Member]
Total investment assets at fair value	222.3	230.6
Derivative instruments
Level 1 [Member] | Fixed Maturities [Member]
Fixed maturities	94.6	124.0
Level 1 [Member] | Equity Securities [Member]
Equity securities	127.7	106.6
Level 2 [Member]
Total investment assets at fair value	4,556.2	4,619.8
Derivative instruments	4.7
Level 2 [Member] | Fixed Maturities [Member]
Fixed maturities	4,531.3	4,609.3
Level 2 [Member] | Equity Securities [Member]
Equity securities	24.9	10.5
Level 3 [Member]
Total investment assets at fair value	74.9	67.5
Level 3 [Member] | Fixed Maturities [Member]
Fixed maturities	72.0	64.6
Level 3 [Member] | Equity Securities [Member]
Equity securities	2.9	2.9
Fixed Maturities [Member]
Fixed maturities	4,697.9	4,797.9
Equity Securities [Member]
Equity securities	$ 155.5	$ 120.0