Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss)
Balance at beginning of period at Dec. 31, 2017		$ 2.9	$ 8.5	$ 2,110.6	$ (25.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			3.8
Stock-based compensation			1.4
Net earnings	$ 174.3			174.3
Dividends paid in cash				(106.4)
Other comprehensive income					2.0
Balance at end of period at Mar. 31, 2018	$ 2,172.0	2.9	13.7	2,178.5	(23.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.37
Balance at beginning of period at Dec. 31, 2018	$ 2,302.7	2.9	3.0	2,341.6	(44.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			18.1
Stock-based compensation			1.6
Net earnings	194.1			194.1
Dividends paid in cash				(123.0)
Other comprehensive income					3.7
Balance at end of period at Mar. 31, 2019	$ 2,397.2	$ 2.9	$ 22.7	$ 2,412.7	$ (41.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.43