Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive (loss) income
Balance at beginning of period at Dec. 31, 2018		$ 2.9	$ 3.0	$ 2,341.6	$ (44.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			43.0
Purchases of common stock			0.0
Stock-based compensation			4.3
Net earnings	$ 612.2			612.2
Dividends paid in cash				(372.3)
Other comprehensive (loss) income					(4.2)
Balance at end of period at Sep. 30, 2019	$ 2,585.7	2.9	50.3	2,581.5	(49.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.650
Balance at beginning of period at Dec. 31, 2018		2.9	3.0	2,341.6	(44.8)
Balance at end of period at Dec. 31, 2019	$ 2,665.6	2.9	67.2	2,633.9	(38.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.870
Balance at beginning of period at Jun. 30, 2019		2.9	46.0	2,494.2	(39.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			2.9
Purchases of common stock			0.0
Stock-based compensation			1.4
Net earnings	$ 213.5			213.5
Dividends paid in cash				(126.2)
Other comprehensive (loss) income					(9.6)
Balance at end of period at Sep. 30, 2019	$ 2,585.7	2.9	50.3	2,581.5	(49.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.220
Balance at end of period at Dec. 31, 2019	$ 2,665.6	2.9	67.2	2,633.9	(38.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.220
Stock options exercised			38.3
Purchases of common stock			(52.0)
Stock-based compensation			4.3
Net earnings	$ 663.0			663.0
Dividends paid in cash				(430.2)
Other comprehensive (loss) income					(3.4)
Balance at end of period at Sep. 30, 2020	$ 2,885.6	2.9	57.8	2,866.7	(41.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.750
Balance at beginning of period at Jun. 30, 2020		2.9	44.4	2,788.6	(52.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			12.0
Purchases of common stock			0.0
Stock-based compensation			1.4
Net earnings	$ 221.5			221.5
Dividends paid in cash				(143.4)
Other comprehensive (loss) income					11.1
Balance at end of period at Sep. 30, 2020	$ 2,885.6	$ 2.9	$ 57.8	$ 2,866.7	$ (41.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.250