Investments (Details 9) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 12,288.7	$ 11,868.1	$ 11,802.9
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11,179.8	10,723.8	10,596.8
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	9,942.7	9,284.6
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,038.0	1,219.5
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	152.8	148.3
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	46.3	71.4
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	10,864.2	10,374.9
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	9,755.2	9,115.8
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	910.7	1,041.0
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	152.7	148.3
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	45.6	69.8
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	315.6	348.9
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	187.5	168.8
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	127.3	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	1,108.9	1,144.3	$ 1,206.1
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,085.3	1,116.0
Residential mortgage loans | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 23.6	$ 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	$ 234.1	$ 283.4
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	222.2	268.4
Home equity | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	11.9	15.0
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	874.8	860.9
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	863.1	847.6
First liens | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 11.7	$ 13.3