Interim financial statements - Cumulative Effect Adjustments on Adoption of New Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized investment gains related to equity securities reclassified	$ 2,177	$ 0	$ 1,802	$ 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			$ (4,600)