Note 15 - Selected Quarterly Financial Data (Unaudited) - Schedule of Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue (less agency commissions)	$ 792	$ 604	$ 451	$ 534	$ 579	$ 517	$ 508	$ 518	$ 2,381	$ 2,122	$ 1,084
Operating income	351	212	65	124	180	139	119	40	752	478	389
Net income (loss) attributable to common stockholders	$ 211	$ 109	$ (2)	$ 40	$ 81	$ 46	$ 31	$ (31)	$ 358	$ 127	$ 211
Basic net income (loss) attributable to common stockholders per share (in dollars per share)	$ 2.24	$ 1.15	$ (0.02)	$ 0.41	$ 0.82	$ 0.46	$ 0.31	$ (0.31)	$ 3.73	$ 1.28	$ 2.39
Net income attributable to common stockholders (in dollars per share)	$ 2.22	$ 1.14	$ (0.02)	$ 0.40	$ 0.81	$ 0.46	$ 0.31	$ (0.31)	$ 3.69	$ 1.27	$ 2.37