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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue (less agency commissions)	$ 579	$ 517	$ 508	$ 518	$ 329	$ 279	$ 250	$ 226	$ 2,122	$ 1,084	$ 883
Operating income	180	139	119	40	$ 151	$ 108	$ 80	$ 50	478	389	290
Net (loss) income available to common stockholders	$ 81	$ 46	$ 31	$ (31)					$ 127	$ 211	$ 262
Net income attributable to common stockholders (in dollars per share)	$ 0.82	$ 0.46	$ 0.31	$ (0.31)	$ 1.01	$ 0.71	$ 0.46	$ 0.22	$ 1.28	$ 2.39	$ 3.59
Diluted net (loss) income available to common stockholders per share (in dollars per share)	$ 0.81	$ 0.46	$ 0.31	$ (0.31)	$ 1.00	$ 0.70	$ 0.46	$ 0.22	$ 1.27	$ 2.37	$ 3.55
Net income					$ 88	$ 62	$ 41	$ 20	$ 179	$ 211	$ 262