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 year at Dec. 31, 2017		$ 2.9	$ 8.5	$ 2,110.6	$ (25.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			13.4
Purchases of common stock			(24.0)	(79.0)
Stock-based compensation			5.1
Net earnings	$ 751.9			751.9
Dividends paid in cash				(441.9)
Other comprehensive income (loss)					(19.7)
Balance at end of year at Dec. 31, 2018	$ 2,302.7	2.9	3.0	2,341.6	(44.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 0.77
Stock options exercised			58.5
Purchases of common stock			0.0	0.0
Stock-based compensation			5.7
Net earnings	$ 790.9			790.9
Dividends paid in cash				(498.6)
Other comprehensive income (loss)					6.4
Balance at end of year 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)	$ 0.87
Stock options exercised			41.0
Purchases of common stock			(52.0)	0.0
Stock-based compensation			5.7
Net earnings	$ 859.1			859.1
Dividends paid in cash				(803.4)
Other comprehensive income (loss)					17.2
Balance at end of year at Dec. 31, 2020	$ 2,733.2	$ 2.9	$ 61.9	$ 2,689.6	$ (21.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends paid per share of common stock (in dollars per share)	$ 1.40