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