Condensed Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals) - USD ($) $ / shares in Thousands, $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Accounts receivable, allowance for credit losses	$ 10	$ 10
Series A Perpetual Preferred Stock, par value (in dollars per share)	$ 0	$ 0
Series A Perpetual Preferred Stock, shares authorized (in shares)	1,500,000	1,500,000
Series A Perpetual Preferred Stock, shares issued (in shares)	650,000	650,000
Series A Perpetual Preferred Stock, shares outstanding (in shares)	650,000	650,000
Series A Perpetual Preferred Stock, liquidation value	$ 650	$ 650
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	103,847,337	103,100,856
Common stock, shares outstanding (in shares)	88,628,868	88,140,259
Treasury stock (in shares)	15,218,469	14,960,597
Common Class A [Member]
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	25,000,000	25,000,000
Common stock, shares issued (in shares)	9,169,198	8,935,773
Common stock, shares outstanding (in shares)	7,171,019	7,048,006
Treasury stock (in shares)	1,998,179	1,887,767