Business combination (Acquisition details) (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 02, 2014	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2014
Liabilities acquired
Goodwill		$ 196,758	$ 196,758	$ 195,897
Western World Insurance Group, Inc
Total allocable purchase price
Total allocable purchase price	$ 692,305
Tangible assets acquired
Cash and cash equivalents	48,760
Investments	1,126,185
Receivables	37,848
Other assets	19,418
Tangible assets acquired	1,232,211
Intangible assets acquired
Intangible assets acquired	25,055
Liabilities acquired
Net loss reserves and paid losses recoverable	523,315
Unearned premiums, net of expenses	135,686
Other liabilities	81,464
Liabilities acquired	740,465
Goodwill		176,365		$ 175,504
Measurement period adjustments to assets		$ 861
Western World Insurance Group, Inc | Brand names
Intangible assets acquired
Intangible asset - finite-lived	5,756
Western World Insurance Group, Inc | Distribution network
Intangible assets acquired
Intangible asset - finite-lived	4,651
Western World Insurance Group, Inc | Technology
Intangible assets acquired
Intangible asset - finite-lived	2,323
Western World Insurance Group, Inc | State licenses
Intangible assets acquired
Intangible asset - indefinite-lived	$ 12,325