Business combinations - Selected Unaudited Pro-forma Results of Operations (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total revenues:
Total revenues, As reported	$ 326.3	$ 332.8	$ 350.2	$ 337.1	$ 304.5	$ 314.9	$ 320.9	$ 309.4	$ 1,346.4	$ 1,249.7	$ 1,145.6
Total revenues, Pro forma									1,346.4	1,326.7
Loss from continuing operations before income taxes:
Loss from continuing operations, As reported									(120.8)	(103.2)	$ (31.7)
Loss from continuing operations, Pro forma									$ (120.8)	$ (112.6)