Interim financial statements (Recently Adopted Accounting Pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized investment gains related to equity securities reclassified	$ (2,203)	$ 0
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to retained earnings, tax effect	(1,000)
Retained earnings | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized investment gains related to equity securities reclassified	4,600
AOCI | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized investment gains related to equity securities reclassified	$ (5,800)