Consolidated Statements of Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
finance charges revenue	$ 303.0	$ 259.4	$ 863.0	$ 749.2
Revenue:
Premiums earned	12.2	10.3	34.2	30.9
Other income	16.8	14.2	45.8	42.6
Total revenue	332.0	283.9	943.0	822.7
Costs and expenses:
Salaries and wages	41.1	33.7	123.3	101.9
General and administrative	14.1	14.2	41.3	42.1
Sales and marketing	16.3	14.2	51.3	43.7
Provision for credit losses	14.0	25.7	39.2	68.0
Interest	41.1	30.5	114.3	88.0
Provision for claims	7.0	5.5	19.5	17.6
Total costs and expenses	133.6	123.8	388.9	361.3
Income before provision for income taxes	198.4	160.1	554.1	461.4
Provision for income taxes	47.4	59.4	132.0	168.3
Net income	$ 151.0	$ 100.7	$ 422.1	$ 293.1
Net income per share:
Basic (in usd per share)	$ 7.76	$ 5.19	$ 21.69	$ 15.01
Diluted (in usd per share)	$ 7.75	$ 5.19	$ 21.68	$ 14.99
Weighted average shares outstanding:
Basic (in shares)	19,465,563	19,407,344	19,456,389	19,528,175
Diluted (in shares)	19,473,978	19,415,545	19,472,197	19,547,674