Investments - Loans by Loan to Value (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
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%	100.00%
Total commercial mortgage loans	$ 4,030.1	$ 3,730.3
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%
Commercial mortgage loans	$ 363.3	$ 395.1
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%
Commercial mortgage loans	$ 1,045.1	$ 969.4
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%
Commercial mortgage loans	$ 2,381.9	$ 2,158.2
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%
Commercial mortgage loans	$ 237.4	$ 204.8
80% and above
Schedule of Loans by Loan to Value Ratio [Line Items]
Loan to Value Ratio, minimum	80.00%	80.00%
Commercial mortgage loans	$ 2.4	$ 2.8