Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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, 2015	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
Exercised	(794	)	$	16.07		$	14.4
Outstanding at December 31, 2016	473		$	17.47	0.1	$	10.4
Exercised	(473	)	$	17.47		$	10.4
Outstanding at December 31, 2017	—		$	—	0.0	$	—
Options exercisable at December 31, 2017	—		$	—	0.0	$	—

Cash Proceeds Received and Tax Benefit from Share-based Payment Awards [Table Text Block]

	Year Ended December 31,
(millions of dollars)	2017		2016		2015
Proceeds from stock options exercised — gross	$	8.3	$	12.7	$	6.5
Tax benefit	8.2		0.3		10.3
Proceeds from stock options exercised, net of tax	$	16.5	$	13.0	$	16.8

Restricted stock compensation expense [Text Block]

	Year Ended December 31,
(millions of dollars, except per share data)	2017		2016		2015
Restricted stock compensation expense	$	27.0	$	26.7	$	28.0
Restricted stock compensation expense, net of tax	$	19.7	$	19.5	$	20.4

Status of nonvested restricted stock [Text Block]

	Shares subject to restriction (thousands)		Weighted average grant date fair value
Nonvested at January 1, 2015	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
Granted	724		$	30.07
Vested	(551	)	$	47.55
Forfeited	(70	)	$	43.05
Nonvested at December 31, 2016	1,429		$	44.12
Granted	804		$	40.10
Vested	(521	)	$	56.53
Forfeited	(119	)	$	38.97
Nonvested at December 31, 2017	1,593		$	38.86

Share-based Compensation, Performance Shares Award Nonvested Activity [Table Text Block]

	Year Ended December 31,
(millions of dollars, except share data)	2017		2016		2015
Expense	$	9.9	$	9.6	$	12.2
Number of shares	—		—		—

	Year Ended December 31,
(millions of dollars, except share data)	2017		2016
Expense	$	15.9	$	7.1
Number of shares	126,000		—

A summary of the status of the Company’s nonvested relative revenue growth performance shares at December 31, 2017 and 2016 is as follows:

	Number of shares (thousands)		Weighted average grant date fair value
Nonvested at December 31, 2015	—		$	—
Granted	485		$	38.62
Vested	(126	)	$	38.62
Forfeited	(39	)	$	38.62
Nonvested at December 31, 2016	320		$	38.62
Granted	198		$	40.08
Vested	(156	)	$	38.62
Forfeited	(7	)	$	39.20
Nonvested at December 31, 2017	355		$	39.42