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		(148)
Common stock repurchased, shares	(725,220)	(725,220)
Restricted stock units converted to common stock, shares		21,971
Balance, shares at Sep. 30, 2020		17,649,382
Balance at Dec. 31, 2019	$ 2,355,300,000	$ 200,000	$ 157,700,000	$ 2,196,600,000	$ 800,000
Net income	254,700,000	0	0	254,700,000	0
Other comprehensive loss	900,000	0	0	0	900,000
Stock-based compensation	4,700,000	0	4,700,000	0	0
Common stock repurchased, value	(307,100,000)	0	(2,000,000.0)	(305,100,000)	0
Balance at Sep. 30, 2020	2,308,500,000	$ 200,000	160,400,000	2,146,200,000	1,700,000
Balance, shares at Jun. 30, 2020		17,649,470
Restricted stock awards, net of forfeitures, shares		(88)
Balance, shares at Sep. 30, 2020		17,649,382
Balance at Jun. 30, 2020	2,065,100,000	$ 200,000	158,900,000	1,904,100,000	1,900,000
Net income	242,100,000	0	0	242,100,000	0
Other comprehensive loss	(200,000)	0	0	0	(200,000)
Stock-based compensation	1,500,000	0	1,500,000	0	0
Balance at Sep. 30, 2020	$ 2,308,500,000	$ 200,000	160,400,000	2,146,200,000	1,700,000
Balance, shares at Dec. 31, 2020	17,092,432	17,092,432
Restricted stock awards, net of forfeitures, shares		(109,054)
Common stock repurchased, shares	(2,277,817)	(2,277,817)
Restricted stock units converted to common stock, shares		11,416
Balance, shares at Sep. 30, 2021	14,716,977	14,716,977
Balance at Dec. 31, 2020	$ 2,302,500,000	$ 200,000	161,900,000	2,138,800,000	1,600,000
Net income	740,700,000	0	0	740,700,000	0
Other comprehensive loss	(900,000)	0	0	0	(900,000)
Stock-based compensation	5,400,000	0	5,400,000	0	0
Common stock repurchased, value	(1,093,900,000)	(100,000)	(1,300,000)	(1,092,500,000)	0
Balance at Sep. 30, 2021	$ 1,953,800,000	$ 100,000	166,000,000.0	1,787,000,000	700,000
Balance, shares at Jun. 30, 2021		16,003,249
Restricted stock awards, net of forfeitures, shares		(26)
Common stock repurchased, shares		(1,286,246)
Balance, shares at Sep. 30, 2021	14,716,977	14,716,977
Balance at Jun. 30, 2021	$ 2,393,000,000	$ 200,000	150,800,000	2,241,100,000	900,000
Net income	250,000,000.0	0	0	250,000,000.0	0
Other comprehensive loss	(200,000)	0	0	0	(200,000)
Stock-based compensation	15,200,000	0	15,200,000	0	0
Common stock repurchased, value	(704,200,000)	(100,000)	0	(704,100,000)	0
Balance at Sep. 30, 2021	$ 1,953,800,000	$ 100,000	$ 166,000,000.0	$ 1,787,000,000	$ 700,000