Share Capital (Forward Contracts to Purchase Common Shares) (Details) - Forward Contracts - USD ($) $ / shares in Units, shares in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Common Shares Purchased (shares)		25.8	25.8
Strike Price (in usd per share)	$ 12.00
Total Treasury Shares Recorded		$ 495,000,000.0	$ 495,000,000.0
Total Liability Recorded		309,000,000.0	496,500,000
$200 million - entered on May 13, 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)		9.8	9.8
Strike Price (in usd per share)		$ 12.00	$ 20.51
Total Treasury Shares Recorded		$ 198,700,000	$ 198,700,000
Total Liability Recorded			199,100,000
$200 million - entered on July 2, 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	$ 20.00
Total Treasury Shares Recorded		$ 193,700,000	$ 193,700,000
Total Liability Recorded		119,400,000	195,300,000
$100 million - entered on August 27, 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	$ 16.59
Total Treasury Shares Recorded		$ 102,600,000	$ 102,600,000
Total Liability Recorded		$ 72,600,000	$ 102,100,000