Unaudited Quarterly Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended			3 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013	Jun. 30, 2013	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013
Unaudited Quarterly Data [Line Items]
Revenues	$ 764,363	$ 768,426	$ 828,473	$ 1,596,899	$ 2,361,262	$ 3,157,796	$ 3,094,018	$ 3,075,671
Gross Profit	539,758	537,024	562,550	1,099,574	1,639,332	2,184,363	2,153,493	2,137,464
Operating profit	45,920	40,390	59,458	99,848	145,768	193,462	247,009	313,029
Net earnings	25,925	17,681	27,266	44,947	70,872	96,422	128,757	180,018
Basic earnings per common share	$ 0.49	$ 0.33	$ 0.52	$ 0.85	$ 1.34	$ 1.82	$ 2.35	$ 3.06
Diluted earnings per common share	$ 0.49	$ 0.33	$ 0.51	$ 0.84	$ 1.33	$ 1.81	$ 2.33	$ 3.03
Cash dividends declared per common share						$ 0.93	$ 0.86	$ 0.69
First Quarter [Member]
Unaudited Quarterly Data [Line Items]
Revenues			828,473						815,661
Gross Profit			562,550						550,077
Operating profit			59,458						77,019
Net earnings			27,266						44,987
Basic earnings per common share			$ 0.52						$ 0.78
Diluted earnings per common share			$ 0.51						$ 0.77
Cash dividends declared per common share			$ 0.23						$ 0.21
Second Quarter [Member]
Unaudited Quarterly Data [Line Items]
Revenues		768,426								759,424
Gross Profit		537,024								532,949
Operating profit		40,390								78,922
Net earnings		17,681								42,975
Basic earnings per common share		$ 0.33								$ 0.78
Diluted earnings per common share		$ 0.33								$ 0.78
Cash dividends declared per common share		$ 0.23								$ 0.21
Third Quarter [Member]
Unaudited Quarterly Data [Line Items]
Revenues	764,363										752,758
Gross Profit	539,758										530,514
Operating profit	45,920										56,399
Net earnings	25,925										27,558
Basic earnings per common share	$ 0.49										$ 0.52
Diluted earnings per common share	$ 0.49										$ 0.51
Cash dividends declared per common share	$ 0.23										$ 0.21
Fourth Quarter [Member]
Unaudited Quarterly Data [Line Items]
Revenues												796,534	766,175
Gross Profit												545,031	539,953
Operating profit												47,694	34,669
Net earnings												$ 25,550	$ 13,237
Basic earnings per common share												$ 0.48	$ 0.25
Diluted earnings per common share												$ 0.48	$ 0.25
Cash dividends declared per common share												$ 0.24	$ 0.23