Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 4,849.1	$ 4,255.6
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 951.0	$ 968.8
Loans by region percentage of total loans	19.50%	22.90%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,026.0	$ 956.3
Loans by region percentage of total loans	21.20%	22.50%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 513.9	$ 431.9
Loans by region percentage of total loans	10.60%	10.10%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 1,060.3	$ 710.1
Loans by region percentage of total loans	21.90%	16.70%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 469.6	$ 447.8
Loans by region percentage of total loans	9.70%	10.50%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 449.1	$ 370.7
Loans by region percentage of total loans	9.30%	8.70%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 233.5	$ 223.2
Loans by region percentage of total loans	4.80%	5.20%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 64.9	$ 65.7
Loans by region percentage of total loans	1.30%	1.50%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 80.8	$ 81.1
Loans by region percentage of total loans	1.70%	1.90%