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, 2019		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		17,649,470
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 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		17,649,470
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 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
Balance, shares at Dec. 31, 2020	17,092,432	17,092,432
Restricted stock awards, net of forfeitures, shares		(109,028)
Common stock repurchased, shares	(991,571)	(991,571)
Restricted stock units converted to common stock, shares		11,416
Balance, shares at Jun. 30, 2021	16,003,249	16,003,249
Balance at Dec. 31, 2020	$ 2,302,500,000	$ 200,000	161,900,000	2,138,800,000	1,600,000
Net income	490,700,000	0	0	490,700,000	0
Other comprehensive loss	(700,000)	0	0	0	(700,000)
Stock-based compensation	(9,800,000)	0	(9,800,000)	0	0
Common stock repurchased, value	(389,700,000)	0	(1,300,000)	(388,400,000)	0
Balance at Jun. 30, 2021	$ 2,393,000,000.0	$ 200,000	150,800,000	2,241,100,000	900,000
Balance, shares at Mar. 31, 2021		16,710,422
Restricted stock awards, net of forfeitures, shares		(109,010)
Common stock repurchased, shares		(598,163)
Balance, shares at Jun. 30, 2021	16,003,249	16,003,249
Balance at Mar. 31, 2021	$ 2,369,800,000	$ 200,000	161,700,000	2,207,000,000.0	900,000
Net income	288,600,000	0	0	288,600,000	0
Stock-based compensation	(10,900,000)	0	(10,900,000)	0	0
Common stock repurchased, value	(254,500,000)	0	0	(254,500,000)	0
Balance at Jun. 30, 2021	$ 2,393,000,000.0	$ 200,000	$ 150,800,000	$ 2,241,100,000	$ 900,000