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, 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
Beginning Balance at Dec. 31, 2019	3,071.0	$ 0.9	524.8	2,704.8	(159.5)
Beginning Balance (in shares) at Dec. 31, 2019		94,655
Common stock withheld and retired to cover taxes on vested stock awards	(10.4)	$ 0.0	(0.8)	(9.6)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(107)
Common stock dividends declared	(150.5)	$ 0.0	0.0	(150.5)	0.0
Share-based compensation	18.7	$ 0.0	18.7	0.0	0.0
Share-based compensation expense (in shares)		287
Other	(0.1)	$ 0.0	0.0	(0.1)	0.0
Comprehensive income	204.4	0.0	0.0	198.3	6.1
Ending Balance at Jun. 30, 2020	3,133.1	$ 0.9	542.7	2,742.9	(153.4)
Ending Balance (in shares) at Jun. 30, 2020		94,835
Beginning Balance at Mar. 31, 2020	3,149.9	$ 0.9	534.9	2,771.1	(157.0)
Beginning Balance (in shares) at Mar. 31, 2020		94,848
Common stock withheld and retired to cover taxes on vested stock awards	(10.4)	$ 0.0	(0.8)	(9.6)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(106)
Common stock dividends declared	(75.3)	$ 0.0	0.0	(75.3)	0.0
Share-based compensation	8.6	$ 0.0	8.6	0.0	0.0
Share-based compensation expense (in shares)		93
Comprehensive income	60.3	$ 0.0	0.0	56.7	3.6
Ending Balance at Jun. 30, 2020	$ 3,133.1	$ 0.9	$ 542.7	$ 2,742.9	$ (153.4)
Ending Balance (in shares) at Jun. 30, 2020		94,835