Shareholders' Equity (Preferred Shares Outstanding) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares outstanding	162,600	161,000
Liquidation Preference	$ 4,065,000	$ 4,025,000
Series U Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date	Jun. 15, 2017
Dividend Rate %	5.625%
Preferred stock, shares outstanding		11,500
Liquidation Preference		$ 287,500
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	19,800
Liquidation Preference	$ 495,000	$ 495,000
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	$ 500,000
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	$ 225,000
Series Y Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date	Mar. 17, 2019
Dividend Rate %	6.375%
Preferred stock, shares outstanding		11,400
Liquidation Preference		$ 285,000
Series Z Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date	Jun. 04, 2019
Dividend Rate %	6.00%
Preferred stock, shares outstanding		11,500
Liquidation Preference		$ 287,500
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date	Dec. 02, 2019
Dividend Rate %	5.875%
Preferred stock, shares outstanding		7,600
Liquidation Preference		$ 190,000
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	$ 300,000
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	$ 200,000
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	$ 325,000
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	$ 350,000
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	$ 280,000
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	$ 300,000
Series H Preferred Stock [Member]
Class of Stock [Line Items]
Earliest Redemption Date	Mar. 11, 2024
Dividend Rate %	5.60%
Preferred stock, shares outstanding	11,400
Liquidation Preference	$ 285,000
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
Liquidation Preference	$ 316,250
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
Liquidation Preference	$ 258,750
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
Liquidation Preference	$ 230,000