Employee Stock Plans (Tables)

12 Months Ended

Dec. 31, 2018

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

Share-based Compensation, Stock Options, Activity [Table Text Block]

The following information relates to the stock option activity for the year ended December 31, 2018:

	Shares		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life		Aggregate Intrinsic Value
Outstanding at December 31, 2017	1,540,369		$	58.34
Granted	332,928			81.88
Exercised	(176,783	)		49.86
Forfeited	(64,537	)		67.79
Outstanding at December 31, 2018	1,631,977			63.69	81	months	$	14,401
Exercisable at December 31, 2018	787,871		$	54.75	65	months	$	11,579

The aggregate intrinsic value in the table above represents the total pre-tax intrinsic value (the difference between the company's closing stock price on the last trading day of 2018 and the exercise price, multiplied by the number of in-the-money options) received by the option holders had all option holders exercised their options on December 31, 2018. This amount changes based on the market value of the company's stock.

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

The fair value of stock options was estimated using the Black-Scholes valuation model with the following weighted-average assumptions for the years ended December 31:

	2018	2017	2016
Volatility (percent) (a)	24	26	31
Expected term (in years) (b)	5.5	5.1	5.2
Risk-free interest rate (percent) (c)	2.7	1.9	1.3

(a) Volatility is measured using historical daily price changes of the company's common stock over the expected term of the option.

(b) The expected term represents the weighted-average period the option is expected to be outstanding and is based primarily on the historical exercise behavior of employees.

(c) The risk-free interest rate is based on the U.S. Treasury zero-coupon yield with a maturity that approximates the expected term of the option.

Schedule of Nonvested Share Activity [Table Text Block]

The following information summarizes the changes in non-vested performance shares, performance units, restricted stock, and restricted stock units for 2018:

	Shares		Weighted- Average Grant Date Fair Value
Non-vested shares at December 31, 2017	1,176,965		$	62.60
Granted	538,712			77.97
Vested	(523,477	)		62.49
Forfeited	(99,335	)		66.50
Non-vested shares at December 31, 2018	1,092,865		$	69.87