Fair Values of Financial Instruments - Assets, Liabilities and Temporary Equity Measured at Fair Value on Recurring Basis (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Assets
Available for sale securities	$ 38,895,000	$ 31,197,000
Liabilities
Contingent earn-out obligations	11,466
Fair Value, Measurements, Recurring
Assets
Available for sale securities	38,895,000	31,197,000
Interest rate cap agreements	1,032,000	9,929,000
Liabilities
Contingent earn-out obligations	6,388,000	2,950,000
Temporary equity
Noncontrolling interests subject to put provisions	1,011,360,000	973,258,000
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Available for sale securities	38,895,000	31,197,000
Interest rate cap agreements	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)
Assets
Available for sale securities	0	0
Interest rate cap agreements	1,032,000	9,929,000
Liabilities
Contingent earn-out obligations	0	0
Temporary equity
Noncontrolling interests subject to put provisions	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Available for sale securities	0	0
Interest rate cap agreements	0	0
Liabilities
Contingent earn-out obligations	6,388,000	2,950,000
Temporary equity
Noncontrolling interests subject to put provisions	$ 1,011,360,000	$ 973,258,000