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			5.6
Purchases of common stock			(16.2)	(24.2)
Stock-based compensation			2.5
Net earnings	$ 385.5			385.5
Dividends paid in cash				(212.7)
Other comprehensive income (loss)					(10.8)
Balance at end of period at Jun. 30, 2018	$ 2,226.6	2.9	0.4	2,259.2	(35.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.370
Balance at beginning of period at Mar. 31, 2018		2.9	13.7	2,178.5	(23.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			1.8
Purchases of common stock			(16.2)	(24.2)
Stock-based compensation			1.1
Net earnings	$ 211.2			211.2
Dividends paid in cash				(106.3)
Other comprehensive income (loss)					(12.8)
Balance at end of period at Jun. 30, 2018	$ 2,226.6	2.9	0.4	2,259.2	(35.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.185
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			40.1
Purchases of common stock			0.0	0.0
Stock-based compensation			2.9
Net earnings	398.7			398.7
Dividends paid in cash				(246.1)
Other comprehensive income (loss)					5.4
Balance at end of period at Jun. 30, 2019	$ 2,503.7	2.9	46.0	2,494.2	(39.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.430
Balance at beginning of period at Mar. 31, 2019		2.9	22.7	2,412.7	(41.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock options exercised			22.0
Purchases of common stock			0.0	0.0
Stock-based compensation			1.3
Net earnings	$ 204.6			204.6
Dividends paid in cash				(123.1)
Other comprehensive income (loss)					1.7
Balance at end of period at Jun. 30, 2019	$ 2,503.7	$ 2.9	$ 46.0	$ 2,494.2	$ (39.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per share of common stock (in dollars per share)	$ 0.215