Long-term debt - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 29, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Gain (loss) on interest rate cap agreements due to change in their unrealized fair value	$ 37	$ (478)	$ 819	$ (5,479)
Weighted average effective interest rate at quarter end	5.03%		5.03%
Weighted average effective interest rate during quarter	4.93%		4.92%
Percentage of debt instruments bearing fixed interest rate	47.43%		47.43%
U.S. dialysis and related lab services
Debt Instrument [Line Items]
Letters of credit outstanding	$ 22,621		$ 22,621
DMG
Debt Instrument [Line Items]
Letters of credit outstanding	211		211
Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from Long-term Lines of Credit	275,000
Undrawn revolving credit facilities	1,000,000		1,000,000
Letter of Credit
Debt Instrument [Line Items]
Letters of credit outstanding	14,355		14,355
Interest Rate Cap Agreements Effective September 30, 2016
Debt Instrument [Line Items]
Notional amounts of interest rate agreements	3,500,000		$ 3,500,000
Derivative, expiration date			Jun. 30, 2018
Debt expense recognized			$ 4,140
Interest Rate Cap Agreements Effective June 29, 2018
Debt Instrument [Line Items]
Notional amounts of interest rate agreements	3,500,000		$ 3,500,000
Derivative, effective date			Jun. 29, 2018
Derivative, expiration date			Jun. 30, 2020
Fair value of assets	$ 2,135		$ 2,135
Debt expense recognized			2,163
Gain (loss) on interest rate cap agreements due to change in their unrealized fair value			$ 1,103
London Interbank Offered Rate (LIBOR) | Interest Rate Cap Agreements Effective September 30, 2016
Debt Instrument [Line Items]
LIBOR variable rate	3.50%		3.50%
London Interbank Offered Rate (LIBOR) | Interest Rate Cap Agreements Effective June 29, 2018
Debt Instrument [Line Items]
LIBOR variable rate	3.50%		3.50%
Term Loan A-2
Debt Instrument [Line Items]
Credit Facility, Maximum Borrowing Capacity					$ 995,000
Secured Debt	$ 995,000		$ 995,000			$ 0
Term Loan A-2 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
LIBOR plus interest rate margin			1.00%
Term Loan A
Debt Instrument [Line Items]
Mandatory principal payments	75,000		$ 75,000
Secured Debt	700,000		$ 700,000			775,000
Term Loan A | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
LIBOR plus interest rate margin			2.00%
Term Loan B
Debt Instrument [Line Items]
Mandatory principal payments	26,250		$ 26,250
Secured Debt	3,351,250		$ 3,351,250			$ 3,377,500
Term Loan B | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
LIBOR plus interest rate margin			2.75%
Term Loan A subject to interest rate caps
Debt Instrument [Line Items]
Secured Debt	148,750		$ 148,750
Term Loan A subject to uncapped portion of variability of LIBOR
Debt Instrument [Line Items]
Secured Debt	$ 551,250		$ 551,250
Senior Secured Credit Facilities
Debt Instrument [Line Items]
Weighted average effective interest rate at quarter end	4.80%		4.80%