Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2017	$ 2,182.6	$ 0.9	$ 471.2	$ 1,867.4	$ (156.9)
Beginning Balance (in shares) at Dec. 31, 2017		94,350
Common stock withheld and retired to cover taxes on vested stock awards	(7.7)	$ 0.0	(0.4)	(7.3)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(67)
Common stock dividends declared	(134.2)	$ 0.0	0.0	(134.2)	0.0
Adoption of ASC 606 \| ASU 2014-09	1.6	0.0	0.0	1.6	0.0
Share-based compensation	11.1	$ 0.0	11.1	0.0	0.0
Share-based compensation expense (in shares)		217
Other	0.0	$ 0.0	0.2	(0.2)	0.0
Comprehensive income	334.6	0.0	0.0	326.7	7.9
Ending Balance at Jun. 30, 2018	2,388.0	$ 0.9	482.1	2,054.0	(149.0)
Ending Balance (in shares) at Jun. 30, 2018		94,500
Beginning Balance at Mar. 31, 2018	2,273.8	$ 0.9	476.3	1,949.6	(153.0)
Beginning Balance (in shares) at Mar. 31, 2018		94,348
Common stock withheld and retired to cover taxes on vested stock awards	(7.6)	$ 0.0	(0.4)	(7.2)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(67)
Common stock dividends declared	(74.8)	$ 0.0	0.0	(74.8)	0.0
Share-based compensation	6.0	$ 0.0	6.0	0.0	0.0
Share-based compensation expense (in shares)		219
Other	0.0	$ 0.0	0.2	(0.2)	0.0
Comprehensive income	190.6	0.0	0.0	186.6	4.0
Ending Balance at Jun. 30, 2018	2,388.0	$ 0.9	482.1	2,054.0	(149.0)
Ending Balance (in shares) at Jun. 30, 2018		94,500
Beginning Balance at Dec. 31, 2018	2,672.4	$ 0.9	494.5	2,315.8	(138.8)
Beginning Balance (in shares) at Dec. 31, 2018		94,497
Common stock withheld and retired to cover taxes on vested stock awards	(7.7)	$ 0.0	(0.6)	(7.1)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(83)
Common stock dividends declared	(149.7)	$ 0.0	0.0	(149.7)	0.0
Share-based compensation	19.4	$ 0.0	19.4	0.0	0.0
Share-based compensation expense (in shares)		252
Other	(0.2)	$ 0.0	0.0	(0.2)	0.0
Comprehensive income	387.0	0.0	0.0	380.4	6.6
Ending Balance at Jun. 30, 2019	2,921.2	$ 0.9	513.3	2,539.2	(132.2)
Ending Balance (in shares) at Jun. 30, 2019		94,666
Beginning Balance at Mar. 31, 2019	2,794.6	$ 0.9	501.4	2,427.8	(135.5)
Beginning Balance (in shares) at Mar. 31, 2019		94,494
Common stock withheld and retired to cover taxes on vested stock awards	(7.7)	$ 0.0	(0.6)	(7.1)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(83)
Common stock dividends declared	(74.9)	$ 0.0	0.0	(74.9)	0.0
Share-based compensation	12.5	$ 0.0	12.5	0.0	0.0
Share-based compensation expense (in shares)		255
Other	(0.2)	$ 0.0	0.0	(0.2)	0.0
Comprehensive income	196.9	0.0	0.0	193.6	3.3
Ending Balance at Jun. 30, 2019	$ 2,921.2	$ 0.9	$ 513.3	$ 2,539.2	$ (132.2)
Ending Balance (in shares) at Jun. 30, 2019		94,666

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