Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($)	Total	Common Stock [Member]	Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, shares at Dec. 31, 2018		18,972,558
Restricted stock awards, net of forfeitures, shares		5,087
Common stock repurchased, shares	(268,611)	(268,611)
Restricted stock units converted to common stock, shares		87,842
Balance, shares at Jun. 30, 2019		18,796,876
Balance at Dec. 31, 2018	$ 1,990,900,000	$ 200,000	$ 154,900,000	$ 1,836,100,000	$ (300,000)
Net income	328,800,000	0	0	328,800,000	0
Other comprehensive income (loss)	1,100,000	0	0	0	1,100,000
Stock-based compensation	3,700,000	0	3,700,000	0	0
Common stock repurchased, value	(109,200,000)	0	(4,900,000)	(104,300,000)	0
Balance at Jun. 30, 2019	2,215,300,000	$ 200,000	153,700,000	2,060,600,000	800,000
Balance, shares at Mar. 31, 2019		18,797,071
Restricted stock awards, net of forfeitures, shares		(195)
Balance, shares at Jun. 30, 2019		18,796,876
Balance at Mar. 31, 2019	2,048,900,000	$ 200,000	152,200,000	1,896,200,000	300,000
Net income	164,400,000	0	0	164,400,000	0
Other comprehensive income (loss)	500,000	0	0	0	500,000
Stock-based compensation	1,500,000	0	1,500,000	0	0
Balance at Jun. 30, 2019	$ 2,215,300,000	$ 200,000	153,700,000	2,060,600,000	800,000
Balance, shares at Dec. 31, 2019	18,972,558	18,352,779
Restricted stock awards, net of forfeitures, shares		(60)
Common stock repurchased, shares	(725,220)	(725,220)
Restricted stock units converted to common stock, shares		21,971
Balance, shares at Jun. 30, 2020	17,649,748	17,649,470
Balance at Dec. 31, 2019	$ 2,355,300,000	$ 200,000	157,700,000	2,196,600,000	800,000
Net income	12,600,000	0	0	12,600,000	0
Other comprehensive income (loss)	1,100,000	0	0	0	1,100,000
Stock-based compensation	3,200,000	0	3,200,000	0	0
Common stock repurchased, value	(307,100,000)	0	(2,000,000.0)	(305,100,000)	0
Balance at Jun. 30, 2020	$ 2,065,100,000	$ 200,000	158,900,000	1,904,100,000	1,900,000
Balance, shares at Mar. 31, 2020		17,649,478
Restricted stock awards, net of forfeitures, shares		(8)
Balance, shares at Jun. 30, 2020	17,649,748	17,649,470
Balance at Mar. 31, 2020	$ 1,966,100,000	$ 200,000	157,500,000	1,807,700,000	700,000
Net income	96,400,000	0	0	96,400,000	0
Other comprehensive income (loss)	1,200,000	0	0	0	1,200,000
Stock-based compensation	1,400,000	0	1,400,000	0	0
Balance at Jun. 30, 2020	$ 2,065,100,000	$ 200,000	$ 158,900,000	$ 1,904,100,000	$ 1,900,000