Investments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)
Schedule of Investments [Line Items]
Maximum individual state exposure	0.40%	0.30%
Advance refunded or escrowed-to-maturity securities	$ 44.3	$ 51.9
Percentage of revenue securities	67.00%	60.00%
Fixed maturity securities, fair value	$ 13,449.0	$ 12,322.4
Non-income producing material investments	$ 0.0	$ 0.0	$ 0.0
Securities traded continuously at a price below book value, months	4 months
Percentage of securities representing gross unrealized losses	3.00%	1.00%
Percentage of gross unrealized losses in a continuous loss position less than twelve months	76.00%	49.00%
Individual securities comprising total gross unrealized losses (in shares) | security	230	330
Approximate percentage, outstanding principal balance of commercial mortgage loans	41.00%
Mortgage loan commitments outstanding	$ 7.0
Short term investments and fixed maturities	602.6	$ 594.2
Fixed Maturities
Schedule of Investments [Line Items]
Fixed maturity securities, fair value	13,449.0	12,322.4
Fixed maturity securities available for sale, gross unrealized gains	1,931.7	1,266.0
Fixed maturity, unrealized losses	13.6	8.4
Minimum
Schedule of Investments [Line Items]
Outstanding balance of commercial mortgage loans	0.1	0.1
Maximum
Schedule of Investments [Line Items]
Outstanding balance of commercial mortgage loans	$ 12.1	$ 12.3
Canadian Government/Provincials
Schedule of Investments [Line Items]
Percentage of investments in foreign governments fixed maturity securities	59.00%	58.00%
Governments Of Brazil
Schedule of Investments [Line Items]
Percentage of investments in foreign governments fixed maturity securities	12.00%	20.00%
Governments Of Mexico
Schedule of Investments [Line Items]
Percentage of investments in foreign governments fixed maturity securities	8.00%	6.00%
Other Country
Schedule of Investments [Line Items]
Percentage of investments in foreign governments fixed maturity securities	4.00%	5.00%
Foreign corporate | Energy Sector | Fixed Maturities
Schedule of Investments [Line Items]
Fixed maturity securities, fair value	$ 793.7	$ 784.4
Fixed maturity, unrealized losses	$ 101.2	$ 93.1
Foreign corporate | Energy Sector | Fixed Maturities | Internal Investment Grade
Schedule of Investments [Line Items]
Percentage of European investments held in financial industry classification	85.00%	94.00%
Europe | Corporate Fixed Maturity and Equity Securities
Schedule of Investments [Line Items]
Fixed maturity securities, fair value	$ 900.2	$ 802.3
Fixed maturity securities available for sale, gross unrealized gains	$ 120.9	$ 82.4
Investment Sector Concentration Risk | Investments | Europe | Corporate Fixed Maturity and Equity Securities
Schedule of Investments [Line Items]
Percentage of European investments held in financial industry classification	27.00%	28.00%
Geographic Concentration Risk | Investments | United Kingdom | Corporate Fixed Maturity and Equity Securities
Schedule of Investments [Line Items]
Percentage of European investments held in financial industry classification	4.00%	4.00%
Not Designated as Hedging Instrument
Schedule of Investments [Line Items]
Amounts related to derivative assets	$ 0.4	$ 0.6
Amounts related to derivative liabilities	0.7	101.8
Derivative gains (losses) recorded to income statement	$ 6.6	$ (89.5)	$ 11.0