Equity Incentive Plan (Tables)

12 Months Ended

Dec. 31, 2018

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

Table of Performance Share Unit Monte Carlo Assumptions [Table Text Block]

	2018		2017
Risk-free interest rate	3.03	%	1.73	%
Expected life in years	3.0		3.0
Expected volatility	26	%	28	%
Expected dividend yield	—		—

Schedule of Stock Options, Activity

	Number of Options		Weighted Average Exercise Price	Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2017	1,008,476		$232.99	$392.2
Granted	43,943		595.31
Exercised	(163,120	)	150.81
Forfeited	(41,469	)	369.93
Outstanding at December 31, 2018	847,830		$260.88	$264.0
Options exercisable at December 31, 2018	676,202		$208.01	$242.8

Schedule Shares Outstanding under Stock Option Plans, by Exercise Price Range

Number of Options Outstanding	Weighted Average Exercise Price	Remaining Contractual Life of Options Outstanding	Options Exercisable
74,868	$90.76	0.8	74,868
86,140	$133.00	1.8	86,140
214,451	$159.78	3.3	214,451
113,906	$244.99	4.9	113,906
358,465	$392.68	7.4	186,837
847,830		4.9	676,202

Schedule of Stock Options, Valuation Assumptions

	2018		2017		2016
Risk-free interest rate	3.09	%	2.00	%	1.26	%
Expected life in years	5.9		5.8		5.7
Expected volatility	26	%	28	%	29	%
Expected dividend yield	—		—		—

Schedule Restricted Stock Units, Activity

	Number of Restricted Stock Units		Aggregate Intrinsic Value (in millions)	Number of Performance Share Units		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2017	56,389		$34.9	8,050		$5.0
Granted	16,380			4,912
Vested	(20,259	)		—
Forfeited	(3,258	)		(148	)
Outstanding at December 31, 2018	49,252		$27.8	12,814		$7.2