Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 16,164.9	$ 15,364.3	$ 14,398.0
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	14,830.6	13,996.3	13,071.9
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	13,795.4	12,819.5
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	944.9	1,083.0
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	84.7	88.3
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	5.6	5.5
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	14,660.1	13,806.3
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	13,714.8	12,735.2
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	855.0	977.3
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	84.7	88.3
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	5.6	5.5
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	170.5	190.0
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	80.6	84.3
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	89.9	105.7
Residential mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,334.3	1,368.0	$ 1,326.1
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,319.7	1,351.1
Residential mortgage loans | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 14.6	16.9
Mortgage loans, Days delinquent to be considered non-performing	90 days
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 1,321.6	1,352.9
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,310.4	1,340.3
First liens | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11.2	12.6
Home equity
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	12.7	15.1
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	9.3	10.8
Home equity | Non-performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 3.4	$ 4.3