Derivatives - Underlying CPI Index (Details) € in Millions, £ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) $ / derivative_unit	Dec. 31, 2019 USD ($) $ / derivative_unit	Dec. 31, 2020 EUR (€) $ / derivative_unit	Dec. 31, 2020 GBP (£) $ / derivative_unit	Dec. 31, 2019 EUR (€) $ / derivative_unit	Dec. 31, 2019 GBP (£) $ / derivative_unit
Disclosure of financial assets [line items]
Notional amount	$ 8,849.0	$ 7,261.0
Market value	382.7	288.0
Net gains (losses) on investments	313.1	1,716.2
CPI-linked derivatives
Disclosure of financial assets [line items]
Net gains (losses) on investments	$ (13.9)	$ (12.3)
CPI-linked derivatives
Disclosure of financial assets [line items]
Average life (in years)	2 years 8 months 12 days	2 years 9 months 18 days
Notional amount	$ 74,906.0	$ 99,804.7
Cost	347.5	614.9
Market value	2.8	6.7
Unrealized gain (loss)	$ (344.7)	$ (608.2)
United States \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.50%		0.50%	0.50%
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	245.86	244.63	245.86	245.86	244.63	244.63
United States, 1 \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Average life (in years)	2 years 7 months 6 days	2 years 8 months 12 days
Notional amount	$ 32,175.0	$ 44,775.0
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	232.09	231.35	232.09	232.09	231.35	231.35
Index value at period end (usd per unit) \| $ / derivative_unit	260.47	256.97	260.47	260.47	256.97	256.97
Cost	$ 121.0	$ 277.5
Cost (in bps)	0.376%	0.62%	0.376%	0.376%	0.62%	0.62%
Market value	$ 0.9	$ 1.6
Market value (in bps)	0.003%	0.004%	0.003%	0.003%	0.004%	0.004%
Unrealized gain (loss)	$ (120.1)	$ (275.9)
United States, 2 \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.50%	0.50%	0.50%	0.50%	0.50%	0.50%
Average life (in years)	3 years 9 months 18 days	4 years 9 months 18 days
Notional amount	$ 12,600.0	$ 12,600.0
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	238.30	238.30	238.30	238.30	238.30	238.30
Index value at period end (usd per unit) \| $ / derivative_unit	260.47	256.97	260.47	260.47	256.97	256.97
Cost	$ 39.8	$ 39.7
Cost (in bps)	0.316%	0.315%	0.316%	0.316%	0.315%	0.315%
Market value	$ 1.1	$ 4.4
Market value (in bps)	0.009%	0.035%	0.009%	0.009%	0.035%	0.035%
Unrealized gain (loss)	$ (38.7)	$ (35.3)
European Union \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Average life (in years)	2 years 4 months 24 days	2 years 2 months 12 days
Notional amount	$ 24,226.4	$ 36,509.3	€ 19,800.0		€ 32,525.0
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	98.96	96.57	98.96	98.96	96.57	96.57
Index value at period end (usd per unit) \| $ / derivative_unit	104.70	105.13	104.70	104.70	105.13	105.13
Cost	$ 155.6	$ 263.6
Cost (in bps)	0.642%	0.722%	0.642%	0.642%	0.722%	0.722%
Market value	$ 0.6	$ 0.6
Market value (in bps)	0.002%	0.002%	0.002%	0.002%	0.002%	0.002%
Unrealized gain (loss)	$ (155.0)	$ (263.0)
United Kingdom \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Average life (in years)	2 years 4 months 24 days	2 years 10 months 24 days
Notional amount	$ 2,050.4	$ 2,384.5		£ 1,500.0		£ 1,800.0
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	249.23	243.79	249.23	249.23	243.79	243.79
Index value at period end (usd per unit) \| $ / derivative_unit	295.40	291.90	295.40	295.40	291.90	291.90
Cost	$ 10.4	$ 13.4
Cost (in bps)	0.507%	0.562%	0.507%	0.507%	0.562%	0.562%
Market value	$ 0.0	$ 0.0
Market value (in bps)	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Unrealized gain (loss)	$ (10.4)	$ (13.4)
France \| CPI-linked derivatives
Disclosure of financial assets [line items]
Floor rate	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Average life (in years)	2 years 1 month 6 days	3 years 1 month 6 days
Notional amount	$ 3,854.2	$ 3,535.9	€ 3,150.0		€ 3,150.0
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	99.27	99.27	99.27	99.27	99.27	99.27
Index value at period end (usd per unit) \| $ / derivative_unit	104.09	104.39	104.09	104.09	104.39	104.39
Cost	$ 20.7	$ 20.7
Cost (in bps)	0.537%	0.585%	0.537%	0.537%	0.585%	0.585%
Market value	$ 0.2	$ 0.1
Market value (in bps)	0.005%	0.003%	0.005%	0.005%	0.003%	0.003%
Unrealized gain (loss)	$ (20.5)	$ (20.6)
United States, European Union, and United Kingdom \| CPI-linked derivatives
Disclosure of financial assets [line items]
Notional amount	$ 27,215.3	1,800.3
Payout At Maturity Scenario 1
Disclosure of financial assets [line items]
Floor rate	0.00%		0.00%	0.00%
Payout At Maturity Scenario 2
Disclosure of financial assets [line items]
Floor rate	0.50%		0.50%	0.50%
Weighted average index value or strike price (usd per unit) \| $ / derivative_unit	250.49		250.49	250.49
European Run-off \| CPI-linked derivatives
Disclosure of financial assets [line items]
Notional amount		12,054.3
Market value		$ 0.2

The average strike price on the group of price risk option contracts such as put options or call options.

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Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Floor rate on an interest rate derivative such as an interest rate floor or collar. If market rates falls below the floor rate, a payment or receipt is triggered on the contract.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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The amount of financial assets classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

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The gains (losses) on financial assets at fair value through profit or loss. [Refer: Financial assets at fair value through profit or loss]

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The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

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