Goodwill and other intangible assets (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 196,758	$ 196,758	$ 195,897
Measurement period adjustments		861
Amortization of intangible assets	5,666	5,666	4,538
Western World Insurance Group, Inc
Business Acquisition [Line Items]
Goodwill	$ 176,365	176,365	$ 175,504
Measurement period adjustments		$ 861
Minimum
Business Acquisition [Line Items]
Useful lives of acquired intangible assets	5 years
Maximum
Business Acquisition [Line Items]
Useful lives of acquired intangible assets	10 years