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		4,342
Common stock repurchased, shares	(6,185)	(6,185)
Restricted stock units converted to common stock, shares		1,439
Balance, shares at Jun. 30, 2018		19,309,645
Balance at Dec. 31, 2017	$ 1,535.8	$ 0.2	$ 145.5	$ 1,390.3	$ (0.2)
Net income	271.1	0.0	0.0	271.1	0.0
Other comprehensive income (loss)	(0.4)	0.0	0.0	0.0	(0.4)
Stock-based compensation	5.1	0.0	5.1	0.0	0.0
Common stock repurchased, value	(2.0)	0.0	(0.9)	(1.1)	0.0
Balance at Jun. 30, 2018	1,809.6	$ 0.2	149.7	1,660.3	(0.6)
Balance, shares at Mar. 31, 2018		19,309,937
Restricted stock awards, net of forfeitures, shares		(292)
Balance, shares at Jun. 30, 2018		19,309,645
Balance at Mar. 31, 2018	1,656.7	$ 0.2	147.7	1,509.3	(0.5)
Net income	151.0	0.0	0.0	151.0	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 Jun. 30, 2018	$ 1,809.6	$ 0.2	149.7	1,660.3	(0.6)
Balance, shares at Dec. 31, 2018	18,972,558	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	18,796,876
Balance at Dec. 31, 2018	$ 1,990.9	$ 0.2	154.9	1,836.1	(0.3)
Net income	328.8	0.0	0.0	328.8	0.0
Other comprehensive income (loss)	1.1	0.0	0.0	0.0	1.1
Stock-based compensation	3.7	0.0	3.7	0.0	0.0
Common stock repurchased, value	(109.2)	0.0	(4.9)	(104.3)	0.0
Balance at Jun. 30, 2019	$ 2,215.3	$ 0.2	153.7	2,060.6	0.8
Balance, shares at Mar. 31, 2019		18,797,071
Restricted stock awards, net of forfeitures, shares		(195)
Common stock repurchased, shares	0
Balance, shares at Jun. 30, 2019	18,796,876	18,796,876
Balance at Mar. 31, 2019	$ 2,048.9	$ 0.2	152.2	1,896.2	0.3
Net income	164.4	0.0	0.0	164.4	0.0
Other comprehensive income (loss)	0.5	0.0	0.0	0.0	0.5
Stock-based compensation	1.5	0.0	1.5	0.0	0.0
Balance at Jun. 30, 2019	$ 2,215.3	$ 0.2	$ 153.7	$ 2,060.6	$ 0.8