Stock compensation plans (Tables)

12 Months Ended

Dec. 31, 2018

Dec. 31, 2017

Share-based Compensation [Abstract]

Weighted-Average Assumptions used to Estimate Fair Value of Options Granted

The fair value of options granted in 2018, 2017 and 2016 was estimated at the date of grant using a Black-Scholes option pricing model. The following weighted-average assumptions were used:

	2018		2017		2016
Risk-free interest rate	2.67	%	1.88	%	1.30	%
Expected life of option	4.98 years		4.94 years		4.91 years
Expected dividend yield	0.54	%	0.71	%	0.94	%
Expected volatility	22.65	%	21.74	%	21.64	%

Weighted-Average Assumptions used to Estimate Fair Value of Non-Vested Shares Granted

The following weighted-average assumptions were used:

	2018		2017		2016
Risk-free interest rate	2.41	%	1.47	%	0.94	%
Expected dividend yield	0.53	%	0.71	%	0.93	%

Summary of Stock Option Activity

The following table summarizes the option activity during 2018:

	Shares Subject to Options		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life In Years	Aggregate Intrinsic Value
						(Dollars in thousands)
Outstanding, beginning of the year	1,708,928		$	113.49	—	—
Granted	155,498		254.60		—	—
Exercised	(383,198	)	80.51		—	—
Forfeited or expired	(9,779	)	170.78		—	—
Outstanding, end of the year	1,471,449		136.62		5.7	179,396
Exercisable, end of the year	1,073,198		$	112.13	5.0	157,070

Summary of Non-Vested Restricted Stock Unit Activity

The following table summarizes the non-vested restricted stock unit activity during 2018:

	Number of Non-Vested Shares		Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Life	Aggregate Intrinsic Value
					(In years)	(Dollars in thousands)
Outstanding, beginning of the year	233,742		$	148.79
Granted	62,221		250.66
Vested	(83,396	)	125.70
Forfeited	(10,755	)	179.08
Outstanding, end of the year	201,812		188.10		1.1	$	52,164