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	(10.5)	$ 0.0	(0.8)	(9.7)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(108)
Common stock dividends declared	(225.8)	$ 0.0	0.0	(225.8)	0.0
Share-based compensation and other	24.7	$ 0.0	24.7	0.0	0.0
Share-based compensation expense (in shares)		284
Comprehensive income	346.0	$ 0.0	0.0	337.4	8.6
Ending Balance at Sep. 30, 2020	3,205.4	$ 0.9	548.7	2,806.7	(150.9)
Ending Balance (in shares) at Sep. 30, 2020		94,831
Beginning Balance at Jun. 30, 2020	3,133.1	$ 0.9	542.7	2,742.9	(153.4)
Beginning Balance (in shares) at Jun. 30, 2020		94,835
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)		(1,000)
Common stock dividends declared	(75.2)	$ 0.0	0.0	(75.2)	0.0
Share-based compensation and other	6.0	$ 0.0	6.0	0.0	0.0
Share-based compensation expense (in shares)		3
Comprehensive income	141.6	$ 0.0	0.0	139.1	2.5
Ending Balance at Sep. 30, 2020	3,205.4	$ 0.9	548.7	2,806.7	(150.9)
Ending Balance (in shares) at Sep. 30, 2020		94,831
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	(12.0)	$ (0.1)	(0.7)	(11.2)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(88)
Common stock dividends declared	(286.2)	$ 0.0	0.0	(286.2)	0.0
Share-based compensation and other	27.9	$ 0.2	27.6	0.1	0.0
Share-based compensation expense (in shares)		249
Comprehensive income	632.3	$ 0.0	0.0	624.6	7.7
Ending Balance at Sep. 30, 2021	3,608.3	$ 1.0	581.3	3,162.8	(136.8)
Ending Balance (in shares) at Sep. 30, 2021		94,991
Beginning Balance at Jun. 30, 2021	3,444.3	$ 1.0	574.5	3,008.5	(139.7)
Beginning Balance (in shares) at Jun. 30, 2021		94,979
Common stock withheld and retired to cover taxes on vested stock awards	(1.4)	$ 0.0	(0.1)	(1.3)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(10)
Common stock dividends declared	(95.3)	$ 0.0	0.0	(95.3)	0.0
Share-based compensation and other	7.1	$ 0.0	6.9	0.2	0.0
Share-based compensation expense (in shares)		22
Comprehensive income	253.6	$ 0.0	0.0	250.7	2.9
Ending Balance at Sep. 30, 2021	$ 3,608.3	$ 1.0	$ 581.3	$ 3,162.8	$ (136.8)
Ending Balance (in shares) at Sep. 30, 2021		94,991