Investments - Loans by Loan to Value (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Schedule of Loans by Loan to Value Ratio [Line Items]
Benchmark loan to value ratio, greater than indicates unpaid loan amount exceeds underlying collateral	100.00%
Total commercial mortgage loans	$ 3,189.4	[1]	$ 2,874.0	[1]
0% - 50%
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	0.00%		0.00%
Loan to Value Ratio, maximum	50.00%		50.00%
Total commercial mortgage loans	508.9	[1]	501.3	[1]
50% - 60%
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	50.00%		50.00%
Loan to Value Ratio, maximum	60.00%		60.00%
Total commercial mortgage loans	804.8	[1]	768.9	[1]
60% - 70%
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	60.00%		60.00%
Loan to Value Ratio, maximum	70.00%		70.00%
Total commercial mortgage loans	1,781.1	[1]	1,491.6	[1]
70% - 80%
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	70.00%		70.00%
Loan to Value Ratio, maximum	80.00%		80.00%
Total commercial mortgage loans	79.7	[1]	96.4	[1]
80% and above
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	80.00%		80.00%
Total commercial mortgage loans	$ 14.9	[1]	$ 15.8	[1]

[1] Balances do not include allowance for mortgage loan credit losses.