Capital Structure - Outstanding preferred stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	81,000	92,500
Proceeds from issuance of preferred stock, gross	$ 2,025.0	$ 2,313.0
Aggregate dividend payment	108.0	134.0	$ 134.0
Excess redemption price over carrying value	$ 10.0	$ 37.0	$ 13.0
Series A
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	11,500
Proceeds from issuance of preferred stock, gross	$ 0.0	$ 287.5
Dividend rate	5.625%
Dividends payable, amount per share (in usd per share)	$ 0	$ 1.41	$ 1.41
Aggregate dividend payment	$ 4.0	$ 16.0	$ 16.0
Series C
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	0
Proceeds from issuance of preferred stock, gross	$ 0.0	$ 0.0
Dividend rate	6.75%
Dividends payable, amount per share (in usd per share)	$ 0	$ 0	$ 1.69
Aggregate dividend payment	$ 0.0	$ 0.0	$ 26.0
Series D
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	0
Proceeds from issuance of preferred stock, gross	$ 0.0	$ 0.0
Dividend rate	6.625%
Dividends payable, amount per share (in usd per share)	$ 0	$ 1.66	$ 1.66
Aggregate dividend payment	$ 0.0	$ 9.0	$ 9.0
Series E
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	0
Proceeds from issuance of preferred stock, gross	$ 0.0	$ 0.0
Dividend rate	6.625%
Dividends payable, amount per share (in usd per share)	$ 0	$ 1.66	$ 1.66
Aggregate dividend payment	$ 0.0	$ 49.0	$ 49.0
Series F
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	0
Proceeds from issuance of preferred stock, gross	$ 0.0	$ 0.0
Dividend rate	6.25%
Dividends payable, amount per share (in usd per share)	$ 0	$ 1.56	$ 1.56
Aggregate dividend payment	$ 0.0	$ 16.0	$ 16.0
Series G
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	23,000	23,000
Proceeds from issuance of preferred stock, gross	$ 575.0	$ 575.0
Dividend rate	5.625%
Dividends payable, amount per share (in usd per share)	$ 1.41	$ 1.41	$ 1.41
Aggregate dividend payment	$ 32.0	$ 32.0	$ 18.0
Series H
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	46,000	46,000
Proceeds from issuance of preferred stock, gross	$ 1,150.0	$ 1,150.0
Dividend rate	5.10%
Dividends payable, amount per share (in usd per share)	$ 1.28	$ 1.28	$ 0
Aggregate dividend payment	$ 59.0	$ 12.0	$ 0.0
Series I
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	12,000	12,000
Proceeds from issuance of preferred stock, gross	$ 300.0	$ 300.0
Dividend rate	4.75%
Dividends payable, amount per share (in usd per share)	$ 1.19	$ 1.19	$ 0
Aggregate dividend payment	$ 13.0	$ 0.0	$ 0.0