Investments - Loans by Debt Service Coverage Ratio (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Schedule of Loans by Debt Service Coverage Ratio [Line Items]
Benchmark debt service coverage ratio, less than indicates property's operations income is less than debt payments	100.00%
Total commercial mortgage loans	$ 2,874.0	[1]	$ 2,374.8	[1]
Greater than 1.5x
Schedule of Loans by Debt Service Coverage Ratio [Line Items]
Debt Service Coverage Ratio, minimum	150.00%
Total commercial mortgage loans	2,114.4	[1]	1,600.1	[1]
1.25x - 1.5x
Schedule of Loans by Debt Service Coverage Ratio [Line Items]
Debt Service Coverage Ratio, minimum	125.00%
Debt Service Coverage Ratio, maximum	150.00%
Total commercial mortgage loans	390.5	[1]	408.1	[1]
1.0x - 1.25x
Schedule of Loans by Debt Service Coverage Ratio [Line Items]
Debt Service Coverage Ratio, minimum	100.00%
Debt Service Coverage Ratio, maximum	125.00%
Total commercial mortgage loans	293.1	[1]	286.7	[1]
Less than 1.0x
Schedule of Loans by Debt Service Coverage Ratio [Line Items]
Debt Service Coverage Ratio, minimum	0.00%
Debt Service Coverage Ratio, maximum	100.00%
Total commercial mortgage loans	$ 76.0	[1]	$ 79.9	[1]

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