Fair Values of Financial Instruments - Assets, Liabilities and Temporary Equity Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Investments in equity securities	$ 44,077	$ 39,951
Liabilities
Contingent earn-out obligations	42,378
Fair Value, Measurements, Recurring
Assets
Investments in equity securities	44,077	39,951
Liabilities
Contingent earn-out obligations	30,248	24,586
Temporary equity
Noncontrolling interests subject to put provisions	1,330,028	1,180,376
Fair Value, Measurements, Recurring \| Interest rate cap agreements
Assets
Interest rate cap agreements	2,671	24,452
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1)
Assets
Investments in equity securities	44,077	39,951
Liabilities
Contingent earn-out obligations	0	0
Temporary equity
Noncontrolling interests subject to put provisions	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Interest rate cap agreements
Assets
Interest rate cap agreements	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2)
Assets
Investments in equity securities	0	0
Liabilities
Contingent earn-out obligations	0	0
Temporary equity
Noncontrolling interests subject to put provisions	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Interest rate cap agreements
Assets
Interest rate cap agreements	2,671	24,452
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3)
Assets
Investments in equity securities	0	0
Liabilities
Contingent earn-out obligations	30,248	24,586
Temporary equity
Noncontrolling interests subject to put provisions	1,330,028	1,180,376
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Interest rate cap agreements
Assets
Interest rate cap agreements	$ 0	$ 0

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Amount of liability recognized arising from contingent consideration in a business combination.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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The aggregate fair value as of the reporting date of all noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. This item includes noncontrolling interest holder's ownership (or holders' ownership) regardless of the type of equity interest (common, preferred, other) including all potential organizational (legal) forms of the investee entity.

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