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, 2016	$ 1,759.8	$ 0.9	$ 451.4	$ 1,447.1	$ (139.6)
Beginning Balance (in shares) at Dec. 31, 2016		94,213
Common stock withheld and retired to cover taxes on vested stock awards	(10.8)	$ 0.0	(0.7)	(10.1)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(98)
Common stock dividends declared	(238.2)	$ 0.0	0.0	(238.2)	0.0
Share-based compensation expense	20.6	$ 0.0	20.6	0.0	0.0
Share-based compensation expense (in shares)		235
Other	(0.1)	$ 0.0	(0.1)	0.0	0.0
Comprehensive income	651.3	0.0	0.0	668.6	(17.3)
Ending Balance at Dec. 31, 2017	2,182.6	$ 0.9	471.2	1,867.4	(156.9)
Ending Balance (in shares) at Dec. 31, 2017		94,350
Common stock withheld and retired to cover taxes on vested stock awards	(7.9)	$ 0.0	(0.5)	(7.4)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(69)
Common stock dividends declared	(284.0)	$ 0.0	0.0	(284.0)	0.0
Share-based compensation expense	23.5	$ 0.0	23.5	0.0	0.0
Share-based compensation expense (in shares)		216
Adoption of ASC 606 \| ASU 2014-09	1.6	$ 0.0	0.0	1.6	0.0
Other	0.5	0.0	0.3	0.2	0.0
Comprehensive income	756.1	0.0	0.0	738.0	18.1
Ending Balance at Dec. 31, 2018	2,672.4	$ 0.9	494.5	2,315.8	(138.8)
Ending Balance (in shares) at Dec. 31, 2018		94,497
Common stock withheld and retired to cover taxes on vested stock awards	(8.2)	$ 0.0	(0.7)	(7.5)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(87)
Common stock dividends declared	(299.7)	$ 0.0	0.0	(299.7)	0.0
Share-based compensation expense	31.0	$ 0.0	31.0	0.0	0.0
Share-based compensation expense (in shares)		245
Other	(0.2)	$ 0.0	0.0	(0.2)	0.0
Comprehensive income	675.7	0.0	0.0	696.4	(20.7)
Ending Balance at Dec. 31, 2019	$ 3,071.0	$ 0.9	$ 524.8	$ 2,704.8	$ (159.5)
Ending Balance (in shares) at Dec. 31, 2019		94,655

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