Equity Preferred Stock (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended										12 Months Ended
	Mar. 31, 2019 CAD ($)	Feb. 14, 2019 $ / shares	Dec. 31, 2018 CAD ($)	Nov. 14, 2018 $ / shares	Sep. 30, 2018 CAD ($)	Aug. 14, 2018 $ / shares	Jun. 30, 2018 CAD ($)	May 14, 2018 $ / shares	Mar. 31, 2018 CAD ($)	Feb. 14, 2018 $ / shares	Dec. 31, 2018 CAD ($) $ / shares	Dec. 31, 2017 CAD ($)	Dec. 15, 2017 CAD ($) shares	Dec. 15, 2017 $ / shares	Aug. 15, 2017 CAD ($) shares	Aug. 15, 2017 $ / shares
Class of Stock [Line Items]
Preferred Stock, Value, Issued | $			$ 537.2								$ 537.2	$ 537.2
Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 1 [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid | $ / shares				$ 0.328125		$ 0.328125		$ 0.328125		$ 0.328125
Preferred Stock, Dividends Paid in Cash to the Public | $			$ 7.2		$ 7.2		$ 7.2		$ 6.1
Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 1 [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Dividends Per Share Declared, Preferred Stock | $ / shares		$ 0.328125
Preferred Stock, Dividends Paid in Cash to the Public | $	$ 7.2
Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 3 [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid | $ / shares				$ 0.325		$ 0.325		$ 0.325		$ 0.22082
Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 3 [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Dividends Per Share Declared, Preferred Stock | $ / shares		$ 0.325
Preferred Stock [Member] | Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 1 [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued | shares															12.0
Preferred Stock, Dividends, Per Share, Cash Paid | $ / shares											$ 1.3125
Preferred Stock, Value, Issued | $															$ 300.0
Net Proceeds from Issuance, Preferred Shares | $															292.9
Preferred Stock [Member] | Cumulative Redeemable Minimum Rate Reset Preferred Shares, Series 3 [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued | shares													10.0
Preferred Stock, Dividends, Per Share, Cash Paid | $ / shares											$ 1.3000
Shares Issued, Price Per Share | $ / shares														$ 25.00		$ 25.00
Preferred Stock, Value, Issued | $															$ 250.0
Net Proceeds from Issuance, Preferred Shares | $													$ 243.2