Investment income and other gains and losses - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Proceeds from the sales of investments available-for-sale	$ 76,900,000	$ 81,100,000	$ 69,300,000
Realized gains from sale of debt and equity investments available-for-sale	3,200,000	4,000,000	2,400,000
Net realized losses from change in fair value of contingent consideration liabilities	800,000
Realized loss from sale of a business		3,300,000
Realized gains		1,200,000
Recognized other than temporary impairment	$ 0	0	2,700,000
Impairment of an intangible asset			1,800,000
Gain on sale of office buildings			1,500,000
Facility Closing
Net Investment Income [Line Items]
Charges related to office closures		$ 3,400,000	$ 1,400,000