Shareholders' Equity (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 25, 2021	Nov. 02, 2020	Aug. 06, 2020	Dec. 31, 2019	Dec. 31, 2007
Class of Stock [Line Items]
Common stock - Class A, shares authorized	400,000,000			3,400,000	400,000,000
Common stock - Class A, shares issued	109,819,000				111,775,000
Federal net operating loss carry-forward	$ 770.8
Class A Common Stock [Member] | November 16, 2007 Registration Statement [Member]
Class of Stock [Line Items]
Common stock - Class A, shares authorized						4,000,000.0
Common stock - Class A, shares issued					10,000
Shares reclassified as authorized and unissued	1,200,000
Class A Common Stock [Member] | March 5, 2018 Registration Statement [Member]
Class of Stock [Line Items]
Securities issued	0				0
2020 Plan [Member]
Class of Stock [Line Items]
Common stock - Class A, shares authorized				3,000,000.0
2010 Plan [Member]
Class of Stock [Line Items]
Common stock - Class A, shares authorized				400,000
New Plan [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization		$ 500.0
New Plan [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized			$ 1,000.0
Prior Plan [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization			$ 124.3
Real Estate Investment Trust [Member]
Class of Stock [Line Items]
Federal net operating loss carry-forward	$ 651.1