Segment reporting - Summary of Assets by Segment (Phantom) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Equity method and other investments	$ 240,820	$ 245,534
U.S. dialysis and related lab services
Segment Reporting Information [Line Items]
Equity method and other investments	94,144	84,866
Other—Ancillary services and strategic initiatives
Segment Reporting Information [Line Items]
Equity method and other investments	146,676	160,668
DMG held for sale
Segment Reporting Information [Line Items]
Equity method and other investments	$ 5,060	$ 10,321