Share Capital (Narrative) (Details) CAD / shares in Units, $ / shares in Units, CAD in Millions, $ in Millions			3 Months Ended												12 Months Ended
	Feb. 15, 2018 USD ($) shares	Feb. 14, 2018 $ / shares	Dec. 31, 2017 $ / shares shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Dec. 31, 2016 $ / shares shares	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Dec. 31, 2015 $ / shares CAD / shares shares	Sep. 30, 2015 $ / shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 CAD CAD / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues																123,100,000	98,400,000	98,400,000
Proceeds from Issuance of Common Stock | $																$ 1,129.0		$ 1,088.0
Common Shares Issued under Dividend Reinvestment Plan, Shares															58,480	121,249	10,246,221	10,246,221
Common Shares Issued under Dividend Reinvestment Plan | $															$ 0.6	$ 0.9		$ 73.0
Dividends on Common Shares | $															$ 58.0	$ 52.0		$ 225.0
Common Stock, Dividends, Per Share, Paid | $ / shares			$ 0.015	$ 0.015	$ 0.015	$ 0.015	$ 0.015	$ 0.015	$ 0.015	$ 0.015	$ 0.07	$ 0.07	$ 0.07	$ 0.07	$ 0.06	$ 0.06		$ 0.28
Dividends, Common Shares Issued In Lieu of Cash, Value | $															$ 0.6	$ 0.9		$ 73.0
Common Stock, Capital Shares Reserved for Future Issuance			33,300,000				32,200,000				30,300,000				33,300,000	32,200,000	30,300,000
Subsequent Event [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 400.0
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	35,000,000
Stock Repurchase Program Expiration Date	Feb. 27, 2019
Dividends Payable, Date Declared		Feb. 14, 2018
Common Stock, Dividends, Per Share, Declared | $ / shares		$ 0.015
Dividends Payable, Date to be Paid		Mar. 29, 2018
Dividends Payable, Date of Record		Mar. 15, 2018
Share Offering [Member]
Class of Stock [Line Items]
Shares Issued, Price Per Share | (per share)							$ 9.35				$ 14.60					$ 9.35	CAD 14.60
Stock Issued During Period, Value, New Issues, Gross Proceeds																$ 1,150.0	CAD 1,440	1,130.0
Proceeds from Issuance of Common Stock																$ 1,130.0	CAD 1,390	$ 1,090.0
Initial Allotment [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues																107,000,000	85,616,500	85,616,500
Over-Allotment Option [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues																16,050,000	12,842,475	12,842,475
Restricted Stock Units (RSUs) [Member]
Class of Stock [Line Items]
Years After Grant Date Rights Fully Vest															3 years
TSARs and SARs [Member]
Class of Stock [Line Items]
Years After Grant Date Rights Expire															5 years
TSARs and SARs [Member] | Share-based Compensation Award, Tranche One [Member]
Class of Stock [Line Items]
Vesting Rights Percentage															30.00%
Years After Rights Granted First Portion Of Rights Vest															1 year
TSARs and SARs [Member] | Share-based Compensation Award, Tranche Two [Member]
Class of Stock [Line Items]
Vesting Rights Percentage															30.00%
Years After Grant Date Additional Thirty Percent Granted Vest															2 years
TSARs and SARs [Member] | Share-based Compensation Award, Tranche Three [Member]
Class of Stock [Line Items]
Years After Grant Date Rights Fully Vest															3 years
TSARs and SARs, Granted 2015 [Member]
Class of Stock [Line Items]
Years After Grant Date Rights Expire															7 years