Goodwill and other intangibles - Changes in Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018
Goodwill [Roll Forward]
Balance at beginning of period	$ 231,428	$ 231,428
Acquisitions		11,308
Balance at end of period		242,736
Title
Goodwill [Roll Forward]
Balance at beginning of period	225,699	225,699
Acquisitions	11,300	11,308
Balance at end of period		237,007
Ancillary Services and Corporate
Goodwill [Roll Forward]
Balance at beginning of period	$ 5,729	5,729
Acquisitions		0
Balance at end of period		$ 5,729