Stock compensation plans (Tables)

12 Months Ended

Dec. 31, 2015

Share-based Compensation [Abstract]

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

The fair value of options granted in 2015, 2014 and 2013 was estimated at the date of grant using a Black-Scholes option pricing model. The following weighted-average assumptions were used:

	2015		2014		2013
Risk-free interest rate	1.44	%	1.45	%	0.75	%
Expected life of option	4.87 years		4.89 years		4.87 years
Expected dividend yield	1.12	%	1.34	%	1.73	%
Expected volatility	20.68	%	21.44	%	24.65	%

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

The following weighted-average assumptions were used:

	2015		2014		2013
Risk-free interest rate	0.94	%	0.65	%	0.36	%
Expected dividend yield	1.12	%	1.34	%	1.71	%

Summary of Stock Option Activity

The following table summarizes the option activity during 2015:

	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	1,233,672		$	75.93
Granted	353,688		121.10
Exercised	(112,941	)	68.53
Forfeited or expired	(31,507	)	103.42
Outstanding, end of the year	1,442,912		86.98		7.0	$	63,480
Exercisable, end of the year	839,149		$	71.65	6.0	$	50,180

Summary of Non-Vested Restricted Stock Unit Activity

The following table summarizes the non-vested restricted stock unit activity during 2015:

	Number of Non-Vested Shares		Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Life In Years	Aggregate Intrinsic Value
						(Dollars in thousands)
Outstanding, beginning of the year	313,703		$	76.80
Granted	105,239		118.00
Vested	(106,667	)	61.79
Forfeited	(30,867	)	88.73
Outstanding, end of the year	281,408		96.59		1.2	$	36,818