Stock Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock option activity [Text Block]

	Shares (thousands)		Weighted average exercise price		Weighted average remaining contractual life (in years)	Aggregate intrinsic value (in millions)
Outstanding at January 1, 2013	2,872		$	15.33	3.4	$	58.8
Exercised	(870	)	$	14.17		$	25.5
Forfeited	(5	)	$	17.48
Outstanding at December 31, 2013	1,997		$	15.82	2.6	$	80.0
Exercised	(283	)	$	14.04		$	13.8
Outstanding at December 31, 2014	1,714		$	16.11	1.7	$	66.5
Exercised	(440	)	$	14.76		$	19.2
Forfeited	(7	)	$	14.52
Outstanding at December 31, 2015	1,267		$	16.59	0.9	$	33.7
Options exercisable at December 31, 2015	1,267		$	16.59	0.9	$	33.7

Proceeds from stock option exercises [Text Block]

	Year Ended December 31,
(millions of dollars)	2015		2014		2013
Proceeds from stock options exercised — gross	$	6.5	$	4.0	$	12.3
Tax benefit	10.3		12.9		20.7
Proceeds from stock options exercised, net of tax	$	16.8	$	16.9	$	33.0

Restricted stock compensation expense [Text Block]

	Year Ended December 31,
(millions of dollars, except per share data)	2015		2014		2013
Restricted stock compensation expense	$	28.0	$	20.7	$	25.8
Restricted stock compensation expense, net of tax	$	20.4	$	15.1	$	18.8

Status of nonvested restricted stock [Text Block]

	Shares subject to restriction (thousands)		Weighted average price
Nonvested at January 1, 2013	2,064		$	29.39
Granted	782		$	37.82
Vested	(1,322	)	$	24.36
Forfeited	(113	)	$	36.59
Nonvested at December 31, 2013	1,411		$	37.86
Granted	447		$	54.36
Vested	(530	)	$	37.42
Forfeited	(62	)	$	41.14
Nonvested at December 31, 2014	1,266		$	43.57
Granted	687		$	58.45
Vested	(588	)	$	39.14
Forfeited	(39	)	$	50.85
Nonvested at December 31, 2015	1,326		$	53.18

Amounts expensed under plan and share issuances [Text Block]

	Year Ended December 31,
(millions of dollars, except share data)	2015		2014		2013*
Expense	$	12.2	$	11.4	$	10.8
Number of shares	—		—		545,375

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*Shares were issued in February of the following year.