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, 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	0.0	$ 0.0	0.0	0.0	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(1)
Common stock dividends declared	(75.3)	$ 0.0	0.0	(75.3)	0.0
Share-based compensation	10.1	$ 0.0	10.1	0.0	0.0
Share-based compensation expense (in shares)		194
Other	(0.1)	$ 0.0	0.0	(0.1)	0.0
Comprehensive income	144.2	0.0	0.0	141.7	2.5
Ending Balance at Mar. 31, 2020	3,149.9	$ 0.9	534.9	2,771.1	(157.0)
Ending Balance (in shares) at Mar. 31, 2020		94,848
Beginning Balance at Dec. 31, 2020	3,246.3	$ 0.9	554.4	2,835.5	(144.5)
Beginning Balance (in shares) at Dec. 31, 2020		94,830
Common stock withheld and retired to cover taxes on vested stock awards	(0.1)	$ 0.0	0.0	(0.1)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(1)
Common stock dividends declared	(95.5)	$ 0.0	0.0	(95.5)	0.0
Share-based compensation	11.7	$ 0.1	11.6	0.0	0.0
Share-based compensation expense (in shares)		165
Other	(0.1)	$ 0.0	0.0	(0.1)	0.0
Comprehensive income	168.9	0.0	0.0	166.5	2.4
Ending Balance at Mar. 31, 2021	$ 3,331.2	$ 1.0	$ 566.0	$ 2,906.3	$ (142.1)
Ending Balance (in shares) at Mar. 31, 2021		94,994