Shareholders' Equity (Preferred Shares Outstanding) (Details) - USD ($)			6 Months Ended
	Jun. 17, 2020	Mar. 11, 2019	Jun. 30, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Preferred stock, shares outstanding			165,400	162,600
Liquidation Preference			$ 4,135,000,000	$ 4,065,000,000
Series V Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Sep. 20, 2017
Dividend Rate %			5.375%
Preferred stock, shares outstanding				19,800
Liquidation Preference			$ 495,000,000.0	$ 495,000,000
Series W Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Jan. 16, 2018
Dividend Rate %			5.20%
Preferred stock, shares outstanding			20,000	20,000
Liquidation Preference			$ 500,000,000	$ 500,000,000
Series X Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Mar. 13, 2018
Dividend Rate %			5.20%
Preferred stock, shares outstanding			9,000	9,000
Liquidation Preference			$ 225,000,000	$ 225,000,000
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Jan. 20, 2021
Dividend Rate %			5.40%
Preferred stock, shares outstanding			12,000	12,000
Liquidation Preference			$ 300,000,000	$ 300,000,000
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			May 17, 2021
Dividend Rate %			5.125%
Preferred stock, shares outstanding			8,000	8,000
Liquidation Preference			$ 200,000,000	$ 200,000,000
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Jul. 20, 2021
Dividend Rate %			4.95%
Preferred stock, shares outstanding			13,000	13,000
Liquidation Preference			$ 325,000,000	$ 325,000,000
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Oct. 14, 2021
Dividend Rate %			4.90%
Preferred stock, shares outstanding			14,000	14,000
Liquidation Preference			$ 350,000,000	$ 350,000,000
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Jun. 02, 2022
Dividend Rate %			5.15%
Preferred stock, shares outstanding			11,200	11,200
Liquidation Preference			$ 280,000,000	$ 280,000,000
Series G Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Aug. 09, 2022
Dividend Rate %			5.05%
Preferred stock, shares outstanding			12,000	12,000
Liquidation Preference			$ 300,000,000	$ 300,000,000
Series H Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Mar. 11, 2024
Dividend Rate %		5.60%	5.60%
Preferred stock, shares outstanding			11,400	11,400
Liquidation Preference			$ 285,000,000	$ 285,000,000
Series I Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Sep. 12, 2024
Dividend Rate %			4.875%
Preferred stock, shares outstanding			12,650	12,650
Liquidation Preference			$ 316,250,000	$ 316,250,000
Series J Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Nov. 15, 2024
Dividend Rate %			4.70%
Preferred stock, shares outstanding			10,350	10,350
Liquidation Preference			$ 258,750,000	$ 258,750,000
Series K Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Dec. 20, 2024
Dividend Rate %			4.75%
Preferred stock, shares outstanding			9,200	9,200
Liquidation Preference			$ 230,000,000	$ 230,000,000
Series L Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date			Jun. 17, 2025
Dividend Rate %	4.625%		4.625%
Preferred stock, shares outstanding			22,600
Liquidation Preference			$ 565,000,000