Investments in debt and equity securities - Additional Information (Details) $ in Millions	Sep. 30, 2018 USD ($) investment	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Net unrealized investment gains on equity securities held | $	$ 7.0	$ 5.8
Number of investments in an unrealized loss position	334
Number of investments in an unrealized loss positions for more than 12 months	83