Capital Structure - Outstanding preferred stock (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Nov. 08, 2019	Aug. 08, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Due after ten years			$ 5,141.0	$ 4,641.0
Preferred stock, shares issued (in shares)			92,500	79,800
Aggregate liquidation preference			$ 2,313.0	$ 1,995.0
Aggregate dividend payment			134.0	134.0	$ 116.0
Excess redemption price over carrying value			$ 37.0	$ 13.0
Series A
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			11,500	11,500
Aggregate liquidation preference			$ 287.5	$ 287.5
Dividend rate			5.625%
Dividends payable, amount per share (in usd per share)			$ 1.41	$ 1.41	$ 1.41
Aggregate dividend payment			$ 16.0	$ 16.0	$ 16.0
Series C
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			0	0
Aggregate liquidation preference			$ 0.0	$ 0.0
Dividend rate			6.75%
Dividends payable, amount per share (in usd per share)			$ 0	$ 1.69	$ 1.69
Aggregate dividend payment			$ 0.0	$ 26.0	$ 26.0
Series D
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			0	5,400
Aggregate liquidation preference			$ 0.0	$ 135.0
Dividend rate			6.625%
Dividends payable, amount per share (in usd per share)			$ 1.66	$ 1.66	$ 1.66
Aggregate dividend payment			$ 9.0	$ 9.0	$ 9.0
Series E
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			0	29,900
Aggregate liquidation preference			$ 0.0	$ 747.5
Dividend rate			6.625%
Dividends payable, amount per share (in usd per share)			$ 1.66	$ 1.66	$ 1.66
Aggregate dividend payment			$ 49.0	$ 49.0	$ 49.0
Series F
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			0	10,000
Aggregate liquidation preference			$ 0.0	$ 250.0
Dividend rate			6.25%
Dividends payable, amount per share (in usd per share)			$ 1.56	$ 1.56	$ 1.56
Aggregate dividend payment			$ 16.0	$ 16.0	$ 16.0
Series G
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			23,000	23,000
Aggregate liquidation preference			$ 575.0	$ 575.0
Dividend rate			5.625%
Dividends payable, amount per share (in usd per share)			$ 1.41	$ 1.41	$ 0
Aggregate dividend payment			$ 32.0	$ 18.0	$ 0.0
Series H
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			46,000	0
Aggregate liquidation preference		$ 1,150.0	$ 1,150.0	$ 0.0
Dividend rate		5.10%	5.10%
Dividends payable, amount per share (in usd per share)			$ 1.28	$ 0	$ 0
Aggregate dividend payment			$ 12.0	$ 0.0	$ 0.0
Series I
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			12,000	0
Aggregate liquidation preference	$ 300.0		$ 300.0	$ 0.0
Dividend rate	4.75%		4.75%
Dividends payable, amount per share (in usd per share)			$ 1.19	$ 0	$ 0
Aggregate dividend payment			$ 0.0	$ 0.0	$ 0.0
3.850% Senior Notes, due 2049
Class of Stock [Line Items]
Due after ten years			$ 500.0	$ 0.0