Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2014	$ 1,521.4	$ 1.0	$ 432.1	$ 1,242.2	$ (153.9)
Beginning Balance (in shares) at Dec. 31, 2014		98,368,000
Common stock repurchases and retirements	$ (154.7)	$ 0.0	(15.6)	(139.1)	0.0
Common stock repurchases and retirements (in shares)	(2,326,493)	(2,326,000)
Common stock withheld and retired to cover taxes on vested stock awards	$ (8.7)	$ 0.0	(0.8)	(7.9)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(131,000)
Common stock dividends declared	(214.7)	$ 0.0	0.0	(214.7)	0.0
Restricted stock/performance unit grants and cancellations	6.0	$ 0.0	6.0	0.0	0.0
Restricted stock/performance unit grants and cancellations (in shares)		218,000
Share-based compensation expense	18.2	$ 0.0	18.2	0.0	0.0
Comprehensive income	465.8	0.0	0.0	436.8	29.0
Ending Balance at Dec. 31, 2015	1,633.3	$ 1.0	439.9	1,317.3	(124.9)
Ending Balance (in shares) at Dec. 31, 2015		96,129,000
Common stock repurchases and retirements	$ (100.3)	$ (0.1)	(13.1)	(87.1)	0.0
Common stock repurchases and retirements (in shares)	(1,987,187)	(1,987,000)
Common stock withheld and retired to cover taxes on vested stock awards	$ (11.2)	$ 0.0	(1.1)	(10.1)	0.0
Common stock withheld and retired to cover taxes on vested stock awards (in shares)		(172,000)
Common stock dividends declared	(222.6)	$ 0.0	0.0	(222.6)	0.0
Restricted stock/performance unit grants and cancellations	5.7	$ 0.0	5.7	0.0	0.0
Restricted stock/performance unit grants and cancellations (in shares)		243,000
Share-based compensation expense	19.7	$ 0.0	19.7	0.0	0.0
Other	0.3	0.0	0.3	0.0	0.0
Comprehensive income	434.9	0.0	0.0	449.6	(14.7)
Ending Balance at Dec. 31, 2016	1,759.8	$ 0.9	451.4	1,447.1	(139.6)
Ending Balance (in shares) at Dec. 31, 2016		94,213,000
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)		(98,000)
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 (in shares)		235,000
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		94,350,000