Goodwill and other intangible assets (Analysis of goodwill and intangible assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill, beginning of year	$ 196,758	$ 195,897
Measurement period adjustments		861
Goodwill, end of year	196,758	196,758	$ 195,897
Finite lived intangible assets, beginning of year	17,090	22,756
Finite lived intangible assets, end of year	11,424	17,090	22,756
Indefinite-lived intangible assets, beginning of year	104,168	104,168
Indefinite-lived intangible assets, end of year	104,168	104,168	104,168
Total intangible assets, beginning of year	121,258	126,924
Amortization of intangible assets	(5,666)	(5,666)	(4,538)
Total intangible assets, end of year	115,592	121,258	126,924
Talbot
Goodwill [Line Items]
Goodwill, beginning of year	20,393	20,393
Measurement period adjustments		0
Goodwill, end of year	20,393	20,393	20,393
Western World Insurance Group, Inc
Goodwill [Line Items]
Goodwill, beginning of year	176,365	175,504
Measurement period adjustments		861
Goodwill, end of year	$ 176,365	$ 176,365	$ 175,504