Stock-based compensation plans (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Restricted Stock and Restricted Stock Unit Activity

A summary of restricted stock and restricted stock unit activity follows:

	Restricted Stock Units Outstanding			Weighted- Average Grant Price		Restricted Stock Outstanding			Weighted- Average Grant Price
Unvested at January 1, 2017		739,350		$	111.21		499,324		$	109.92
Granted		235,983			162.57		181,939			162.46
Vested		(481,002	)		114.06		(290,935	)		108.97
Cancelled		(11,774	)		129.15		(16,584	)		128.64
Unvested at December 31, 2017		482,557		$	133.05		373,744		$	135.41

Summary of Stock Option Activity

A summary of stock option activity follows:

				Weighted-Average
	Stock Options Outstanding			Exercise Price		Life (In Years)		Aggregate Intrinsic Value (In thousands)
Outstanding at January 1, 2017		1,594,591		$	139.60
Granted		200			162.57
Exercised		(658,400	)		116.20
Expired		(270,979	)		165.46
Outstanding at December 31, 2017		665,412		$	152.23		0.8	$	18,086
Exercisable at December 31, 2017		665,010		$	152.23		0.8	$	18,072