Stock-based compensation plans (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018		482,557		$	133.05		373,744		$	135.41
Granted		348,512			189.52		—			—
Vested		(265,027	)		127.39		(182,905	)		126.60
Cancelled		(9,167	)		194.45		(11,400	)		138.52
Unvested at December 31, 2018		556,875		$	170.07		179,439		$	144.18

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, 2018		665,412		$	152.23
Granted		116,852			190.78
Exercised		(535,724	)		154.91
Expired		(25,949	)		221.81
Outstanding at December 31, 2018		220,591		$	157.98		5.3	$	2,739
Exercisable at December 31, 2018		103,725		$	121.11		5.3	$	2,737