Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 14,064.1	$ 13,275.1	$ 13,134.5
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	12,773.5	12,055.2	11,919.6
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,564.4	10,771.3
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	975.6	1,110.4
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	162.9	160.5
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	70.6	13.0
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	12,535.0	11,795.2
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,414.2	10,612.8
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	887.7	1,009.8
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	162.9	160.5
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	70.2	12.1
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	238.5	260.0
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	150.2	158.5
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	87.9	100.6
Credit tenant loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	0.4	0.9
Residential mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,290.6	1,219.9	$ 1,214.9
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,269.7	1,199.9
Residential mortgage loans | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 20.9	20.0
Mortgage loans, Days delinquent to be considered non-performing	90 days
Home equity
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 125.7	165.6
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	118.8	156.8
Home equity | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	6.9	8.8
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,164.9	1,054.3
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,150.9	1,043.1
First liens | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 14.0	$ 11.2