Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, shares at Dec. 31, 2017		19,310,049
Restricted stock awards, net of forfeitures, shares		4,311
Common stock repurchased, shares	(6,185)	(6,185)
Restricted stock units converted to common stock, shares		1,439
Balance, shares at Sep. 30, 2018		19,309,614
Balance at Dec. 31, 2017	$ 1,535.8	$ 0.2	$ 145.5	$ 1,390.3	$ (0.2)
Net income	422.1	0.0	0.0	422.1	0.0
Other comprehensive income (loss)	(0.4)	0.0	0.0	0.0	(0.4)
Stock-based compensation	7.8	0.0	7.8	0.0	0.0
Common stock repurchased, value	(2.0)	0.0	(0.9)	(1.1)	0.0
Balance at Sep. 30, 2018	1,963.3	$ 0.2	152.4	1,811.3	(0.6)
Balance, shares at Jun. 30, 2018		19,309,645
Restricted stock awards, net of forfeitures, shares		(31)
Balance, shares at Sep. 30, 2018		19,309,614
Balance at Jun. 30, 2018	1,809.6	$ 0.2	149.7	1,660.3	(0.6)
Net income	151.0	0.0	0.0	151.0	0.0
Other comprehensive income (loss)	0.0	0.0	0.0	0.0	0.0
Stock-based compensation	2.7	0.0	2.7	0.0	0.0
Balance at Sep. 30, 2018	$ 1,963.3	$ 0.2	152.4	1,811.3	(0.6)
Balance, shares at Dec. 31, 2018	18,972,558	18,972,558
Restricted stock awards, net of forfeitures, shares		4,981
Common stock repurchased, shares	(268,611)	(268,611)
Restricted stock units converted to common stock, shares		87,842
Balance, shares at Sep. 30, 2019	18,796,770	18,796,770
Balance at Dec. 31, 2018	$ 1,990.9	$ 0.2	154.9	1,836.1	(0.3)
Net income	494.2	0.0	0.0	494.2	0.0
Other comprehensive income (loss)	1.2	0.0	0.0	0.0	1.2
Stock-based compensation	5.7	0.0	5.7	0.0	0.0
Common stock repurchased, value	(109.2)	0.0	(4.9)	(104.3)	0.0
Balance at Sep. 30, 2019	$ 2,382.8	$ 0.2	155.7	2,226.0	0.9
Balance, shares at Jun. 30, 2019		18,796,876
Restricted stock awards, net of forfeitures, shares		(106)
Common stock repurchased, shares	0
Balance, shares at Sep. 30, 2019	18,796,770	18,796,770
Balance at Jun. 30, 2019	$ 2,215.3	$ 0.2	153.7	2,060.6	0.8
Net income	165.4	0.0	0.0	165.4	0.0
Other comprehensive income (loss)	0.1	0.0	0.0	0.0	0.1
Stock-based compensation	2.0	0.0	2.0	0.0	0.0
Balance at Sep. 30, 2019	$ 2,382.8	$ 0.2	$ 155.7	$ 2,226.0	$ 0.9