CONDENSED CONSOLIDATED STATEMENTS OF INCOME (Unaudited) - USD ($)
shares in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Income Statement [Abstract]        
Net sales $ 1,391,545 $ 1,419,947 $ 4,065,719 $ 3,994,123
Cost of goods sold 1,166,402 1,203,246 3,430,062 3,400,992
Gross profit 225,143 216,701 635,657 593,131
Operating expenses:        
Warehousing, marketing and administrative expenses 172,159 148,831 526,653 438,538
Operating income 52,984 67,870 109,004 154,593
Interest expense, net 5,300 3,992 14,918 11,199
Income before income taxes 47,684 63,878 94,086 143,394
Income tax expense 20,017 23,647 42,594 53,349
Net income $ 27,667 $ 40,231 $ 51,492 $ 90,045
Net income per share - basic: $ 0.74 $ 1.05 $ 1.36 $ 2.32
Average number of common shares outstanding - basic 37,300 38,450 37,724 38,817
Net income per share - diluted: $ 0.74 $ 1.03 $ 1.35 $ 2.29
Average number of common shares outstanding - diluted 37,608 38,884 38,109 39,244
Dividends declared per share $ 0.14 $ 0.14 $ 0.42 $ 0.42