Estimated title losses - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Current year estimated loss (percent)	3.80%	3.80%	3.60%
Increase (decrease) in liability claims provision	$ 24.6
Increase (decrease) in prior year estimates		$ 18.6	$ 38.2
Change in prior years reserve, claims payment experience
Gain (Loss) on Investments [Line Items]
Increase (decrease) in liability claims provision	18.4
Change in prior years reserve, large title claims
Gain (Loss) on Investments [Line Items]
Increase (decrease) in liability claims provision	$ 4.3	$ 3.8	$ 31.7