Share Capital - Forward Contracts to Purchase Common Shares (Details) - Forward contract - USD ($) $ / shares in Units, shares in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Common shares purchased (shares)	16.0	25.8
Value of total treasury shares recorded	$ 296,300,000	$ 495,000,000.0
Derivative liability	(192,300,000)	309,000,000.0
Forward Contract, $200 Million, Entered On May 13th, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Notional value of futures contracts		$ 200,000,000
Common shares purchased (shares)		9.8
Strike price (in usd per share)		$ 12.00	$ 20.51
Value of total treasury shares recorded		$ 198,700,000
Derivative liability		117,000,000.0
Forward Contract, $200 Million, Entered On July 2nd, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Notional value of futures contracts	$ 200,000,000	$ 200,000,000
Common shares purchased (shares)	10.0	10.0
Strike price (in usd per share)	$ 12.00	$ 12.00	20.00
Value of total treasury shares recorded	$ 193,700,000	$ 193,700,000
Derivative liability	120,000,000.0	119,400,000
Forward Contract, $100 Million, Entered On August 27th, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Notional value of futures contracts	$ 100,000,000	$ 100,000,000
Common shares purchased (shares)	6.0	6.0
Strike price (in usd per share)	$ 12.00	$ 12.00	$ 16.59
Value of total treasury shares recorded	$ 102,600,000	$ 102,600,000
Derivative liability	$ 72,300,000	$ 72,600,000

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

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The maximum number of shares that could be issued to net share settle a contract, if applicable. If a contract does not have a fixed or determinable maximum number of shares that may be required to be issued, disclose the fact that a potentially infinite number of shares could be issued to settle the contract.

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