Investments in debt and equity securities available-for-sale - Additional Information (Details) $ in Millions	Mar. 31, 2018 USD ($) investment	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Net unrealized investment gains on equity securities held \| $	$ 3.4	$ 5.8
Number of investments in an unrealized loss position	292
Number of investments in an unrealized loss positions for more than 12 months	33

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant