Bank and Other Notes Payable - Narrative (Details) - USD ($)				12 Months Ended
	Oct. 23, 2015	Mar. 29, 2013	Dec. 08, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Bank and other notes payable
Deferred finance cost				$ (17,838,000)	$ (12,716,000)
Loss on extinguishment of debt	$ 578,000			0	(1,709,000)	$ (1,487,000)
Line of Credit
Bank and other notes payable
Line of credit				$ 1,500,000,000
Extension option term				1 year
Maximum contingent borrowing capacity				$ 2,000,000,000
Line of credit				935,000,000	885,000,000
Deferred finance cost				$ (7,548,000)	$ (10,039,000)
Average interest rate (as a percent)				3.13%	2.40%
Line of Credit | Level 2
Bank and other notes payable
Fair value of line of credit				$ 919,158,000	$ 865,921,000
Line of Credit | LIBOR
Bank and other notes payable
Variable interest rate spread (as a percent)				1.45%
Line of Credit | LIBOR | Low end of range
Bank and other notes payable
Variable interest rate spread (as a percent)				1.30%
Line of Credit | LIBOR | High end of range
Bank and other notes payable
Variable interest rate spread (as a percent)				1.90%
Term Loan
Bank and other notes payable
Debt issued			$ 125,000,000
Term Loan | LIBOR | Low end of range
Bank and other notes payable
Variable interest rate spread (as a percent)			1.95%
Term Loan | LIBOR | High end of range
Bank and other notes payable
Variable interest rate spread (as a percent)			3.20%
Prasada Note
Bank and other notes payable
Debt issued		$ 13,330,000
Interest rate on debt (as a percent)		5.25%
Carrying value of term loan				$ 4,732,000	5,521,000
Estimated fair value of term loan				$ 4,717,000	$ 5,786,000