Investments in debt and equity securities - Additional Information (Details) $ in Millions	Mar. 31, 2019 USD ($) investment	Dec. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Net unrealized investment gains on equity securities held | $	$ 6.6	$ 2.9
Number of investments in an unrealized loss position	151
Number of investments in an unrealized loss positions for more than 12 months	138