Bank and Other Notes Payable: (Details) - USD ($)			9 Months Ended	12 Months Ended
	Mar. 29, 2013	Dec. 08, 2011	Sep. 30, 2015	Dec. 31, 2014
Line of Credit
Mortgage loans payable on real estate
Revolving line of credit			$ 1,500,000,000
Expansion borrowing capacity			2,000,000,000
Outstanding borrowings under the line of credit			$ 1,002,000,000	$ 752,000,000
Line of credit, average interest rate (as a percent)			1.88%	1.89%
Fair value of outstanding line of credit			$ 996,765,000	$ 713,989,000
Line of Credit | Low end of range
Mortgage loans payable on real estate
Interest rate basis			LIBOR
Unsecured term loan
Mortgage loans payable on real estate
Interest rate basis			LIBOR
Amount of additional borrowing		$ 125,000,000
Interest rate (as a percent)			2.53%	2.25%
Debt, fair value			$ 125,438,000	$ 119,780,000
Unsecured term loan | Low end of range
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)		1.95%
Unsecured term loan | High end of range
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)		3.20%
Prasada Note
Mortgage loans payable on real estate
Amount of additional borrowing	$ 13,330,000
Interest rate on debt (as a percent)	5.25%
Debt, carrying value			9,575,000	10,879,000
Debt, fair value			$ 9,680,000	$ 11,178,000
London Interbank Offered Rate (LIBOR) | Line of Credit
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)			1.50%
London Interbank Offered Rate (LIBOR) | Line of Credit | Low end of range
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)			1.375%
London Interbank Offered Rate (LIBOR) | Line of Credit | High end of range
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)			2.00%
London Interbank Offered Rate (LIBOR) | Unsecured term loan
Mortgage loans payable on real estate
Interest rate spread over basis (as a percent)			2.20%