Goodwill and other intangibles - Changes in Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Goodwill [Roll Forward]
Balance at beginning of period		$ 217,094
Acquisitions		14,419
Disposals		(85)
Balance at end of period	$ 231,428	231,428
Title
Goodwill [Roll Forward]
Balance at beginning of period		211,365
Acquisitions	14,400	14,419
Disposals		(85)
Balance at end of period	225,699	225,699
Ancillary Services and Corporate
Goodwill [Roll Forward]
Balance at beginning of period		5,729
Acquisitions		0
Disposals		0
Balance at end of period	$ 5,729	$ 5,729