Investment income and other gains and losses - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Proceeds from the sales of investments available-for-sale	$ 81,100,000	$ 69,300,000	$ 58,100,000
Realized loss from sale of a business	(3,300,000)		3,800,000
Realized gains from sale of debt and equity investments available-for-sale		2,400,000	1,100,000
Realized gains (losses)	1,200,000
Gain on sale of office buildings	400,000	1,500,000
Impairment of an intangible asset	0	1,800,000	1,700,000
Impairment of a cost basis investment			1,000,000
Investment and Other (Losses) Gains
Net Investment Income [Line Items]
Realized gains from sale of debt and equity investments available-for-sale	4,000,000
Recognized other than temporary impairment	0	2,700,000
Realized gain from the reduction in the fair value of contingent consideration liability			5,600,000
Facility Closing
Net Investment Income [Line Items]
Charges related to office closures	$ 3,400,000	$ 1,400,000	$ 1,900,000