Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 12,391.0	$ 11,868.1	$ 11,603.4
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,265.3	10,723.8	10,327.7
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	10,068.2	9,284.6
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,011.9	1,219.5
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	159.7	148.3
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	25.5	71.4
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	10,921.0	10,374.9
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	9,844.2	9,115.8
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	892.4	1,041.0
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	159.6	148.3
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	24.8	69.8
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	344.3	348.9
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	224.0	168.8
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	119.5	178.5
Credit tenant loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	0.1
Credit tenant loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	0.7	1.6
Residential mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,125.7	1,144.3	$ 1,275.7
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,103.6	1,116.0
Residential mortgage loans | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 22.1	$ 28.3
Mortgage loans, Days delinquent to be considered nonperforming	90 days	90 days
Home equity
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 218.8	$ 283.4
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	208.0	268.4
Home equity | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	10.8	15.0
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	906.9	860.9
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	895.6	847.6
First liens | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 11.3	$ 13.3