Investments - Loans by Loan to Value (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
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	$ 4,868.5	$ 4,255.6
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	$ 257.3	$ 369.1
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	$ 1,130.6	$ 1,078.9
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	$ 3,171.6	$ 2,574.1
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%
Commercial mortgage loans	$ 285.9	$ 230.8
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	$ 23.1	$ 2.7