Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 14,183.2	$ 13,275.1	$ 12,391.0
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	12,897.3	12,055.2	11,265.3
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,765.2	10,771.3
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	986.8	1,110.4
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	138.7	160.5
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	6.6	13.0
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	12,665.6	11,795.2
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,636.2	10,612.8
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	884.4	1,009.8
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	138.7	160.5
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	6.3	12.1
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	231.7	260.0
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	129.0	158.5
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	102.4	100.6
Credit tenant loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	0.3	0.9
Residential mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,285.9	1,219.9	$ 1,125.7
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,267.9	1,199.9
Residential mortgage loans | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 18.0	$ 20.0
Mortgage loans, Days delinquent to be considered non-performing	90 days	90 days
Home equity
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 23.0	$ 165.6
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	16.5	156.8
Home equity | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	6.5	8.8
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,262.9	1,054.3
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,251.4	1,043.1
First liens | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 11.5	$ 11.2