Net Income Per Common Share (Tables)

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Computation of net income per common share

(Thousands of dollars except per share data)	Three Months Ended March 31,
	2016		2015
Basic
Average common shares outstanding	91,475,860		92,740,059
Net income	$	147,128	$	131,404
Basic net income per common share	$	1.61	$	1.42
Diluted
Average common shares outstanding	91,475,860		92,740,059
Stock options and other contingently issuable shares (1)	1,517,487		2,005,070
Non-vested restricted stock grants	554,887		533,596
Average common shares outstanding assuming dilution	93,548,234		95,278,725
Net income	$	147,128	$	131,404
Diluted net income per common share	$	1.57	$	1.38

⁽¹⁾  Stock options and other contingently issuable shares excluded 34,208 and 12,414 shares due to their anti-dilutive effect for the three months ended March 31, 2016 and 2015, respectively.