Investment income and other gains and losses - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized gains from the sale of investments available-for-sale	$ 0.4	$ 1.6
Impairment of other intangible asset		$ 1.4

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Name:	us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration