Share Capital - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended	9 Months Ended
	Aug. 23, 2017	Jul. 07, 2017	Sep. 30, 2017	Sep. 30, 2017	Feb. 17, 2017	May 13, 2016
Shareholders Equity Disclosure [Line Items]
Share repurchase program, number of shares repurchased and retired in the period			2,700,000	13,800,000
Share repurchase program, value of shares repurchased and retired during the period			$ 120.9	$ 571.5
Minimum
Stock plans
Award vesting period				3 years
Maximum
Stock plans
Award vesting period				4 years
Settling of employee withholding taxes
Shareholders Equity Disclosure [Line Items]
Share repurchase program, value of shares repurchased and retired during the period			$ 0.0	$ 0.6
Stock options
Stock plans
Options granted (in shares)				900,000
Options, weighted average grant date fair value, per share				$ 6.74
Restricted Stock [Member]
Stock plans
Other awards granted during the period (in shares)				34,650
Aggregate grant date fair value				$ 1.5
Restricted stock units
Stock plans
Other awards granted during the period (in shares)				1,100,000
Aggregate grant date fair value				$ 43.4
Restricted stock units | Three Year Option
Stock plans
Award vesting period				3 years
Restricted stock units | Four Year Option
Stock plans
Award vesting period				4 years
Liability-classed cash units
Stock plans
Other awards granted during the period (in shares)				600,000
Aggregate grant date fair value				$ 22.6
Liability-classed cash units | Three Year Option
Stock plans
Award vesting period				3 years
Liability-classed cash units | Four Year Option
Stock plans
Award vesting period				4 years
Performance Shares
Stock plans
Other awards granted during the period (in shares)				600,000
Aggregate grant date fair value				$ 24.6
Maximum number of performance shares				1,200,000
Performance Shares | Minimum
Stock plans
Pinal payout percentage				0.00%
Performance Shares | Maximum
Stock plans
Pinal payout percentage				200.00%
May 2016 Program
Shareholders Equity Disclosure [Line Items]
Share repurchase program, authorized amount						$ 1,000.0
Share repurchase program, number of shares repurchased and retired in the period				2,600,000
Share repurchase program, value of shares repurchased and retired during the period				$ 100.0
Share repurchase program, remaining authorization					$ 349.0
February 2017 Program
Shareholders Equity Disclosure [Line Items]
Share repurchase program, authorized amount					$ 1,000.0
Share repurchase program, number of shares repurchased and retired in the period			2,700,000	11,200,000
Share repurchase program, value of shares repurchased and retired during the period			$ 120.9	$ 470.9
Share repurchase program, remaining authorization			$ 529.1	529.1
Series D Preferred Stock [Member]
Shareholders Equity Disclosure [Line Items]
Share repurchase program, number of shares repurchased and retired in the period	30,652	27,205
Payments for Repurchase of Preferred Stock and Preference Stock	$ 29.5	$ 25.8
Preferred Stock Redemption Discount				$ 2.4
Preferred Stock, Value, Outstanding	$ 30.6	$ 27.1
Series E Preferred Stock
Shareholders Equity Disclosure [Line Items]
Share repurchase program, number of shares repurchased and retired in the period		329,722
Payments for Repurchase of Preferred Stock and Preference Stock		$ 310.6
Preferred Stock Redemption Discount		13.7
Preferred Stock, Value, Outstanding		$ 324.4
Noncumulative perpetual preferred shares
Shareholders Equity Disclosure [Line Items]
Share repurchase program, number of shares repurchased and retired in the period		46,635
Payments for Repurchase of Preferred Stock and Preference Stock		$ 45.6
Preferred Stock Redemption Premium		(1.9)
Preferred Stock, Value, Outstanding		$ 43.7