Notes and Bonds Payable - Narrative (Details)					1 Months Ended	2 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
		May. 15, 2015 USD ($)	Apr. 24, 2015 USD ($) swap_agreement	Feb. 15, 2013 USD ($) extension_option	Feb. 28, 2014 USD ($) Lender	Feb. 15, 2013 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 swap_agreement	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 26, 2013 USD ($)	Oct. 14, 2011 USD ($) Lender	Dec. 13, 2010 USD ($)
Debt Instrument [Line Items]
Notes and bonds payable							$ 1,431,494,000		$ 1,431,494,000	$ 1,403,692,000
Unamortized discount (premium)	[1]						2,667,000		2,667,000
Unaccreted discounts and amortization costs							(28,000,000)		(27,998,000)		$ (29,638,000)
Number of forward starting swaps | swap_agreement			4					4
Unsecured Credit Facility due 2017 [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Write off of unamortized deferred financing costs						$ 300,000
Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							$ 200,000,000		$ 200,000,000	200,000,000
Weighted average interest rate (LOC)							1.60%		1.60%
Senior notes face value					$ 200,000,000.0
Number of Lenders Term Loan | Lender					9
Contractual interest rates, basis spread on variable rate									1.20%
Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate					1.00%
Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate					1.95%
Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							$ 206,000,000		$ 206,000,000	85,000,000
Unsecured credit facility													$ 700,000,000.0
Number of Lenders of unsecured credit facility | Lender													17
Number of extension options | extension_option				2
Period of extension option				6 months
Extension fees as percentage of aggregate commitments				0.075%
Facility fee on the aggregate amount of commitments									0.20%
Amount outstanding under Unsecured Credit Facility							$ 206,000,000		$ 206,000,000
Weighted average interest rate (LOC)							1.60%		1.60%
Remaining borrowing capacity							$ 494,000,000		$ 494,000,000
Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Facility fee on the aggregate amount of commitments				0.15%
Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Facility fee on the aggregate amount of commitments				0.35%
Deferred finance costs, gross				$ 2,700,000		$ 2,700,000
Senior Notes [Member] | 5.750% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							398,168,000		398,168,000	397,864,000
Senior notes face value							$ 400,000,000		$ 400,000,000	400,000,000				$ 400,000,000.0
Interest rate on senior notes							5.75%		5.75%					5.75%
Unaccreted discount							$ 1,832,000		$ 1,832,000	2,136,000
Unamortized discount (premium)														$ 3,200,000
Interest rate yielded per annum upon issuance														5.855%
Amount of amortization included in interest expense on the Company's consolidated Statements of operations									300,000	300,000	300,000
Senior Notes [Member] | 3.75% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							248,435,000		248,435,000	248,253,000
Senior notes face value							$ 250,000,000		$ 250,000,000	250,000,000		$ 250,000,000.0
Interest rate on senior notes							3.75%		3.75%			3.75%
Unaccreted discount							$ 1,565,000		$ 1,565,000	1,747,000		$ 2,100,000
Interest rate yielded per annum upon issuance												3.849%
Amount of amortization included in interest expense on the Company's consolidated Statements of operations									200,000	200,000	100,000
Senior Notes [Member] | 3.875% Unsecured Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							249,804,000		249,804,000	0
Deferred finance costs, gross			$ 2,300,000
Senior notes face value			$ 250,000,000.0				250,000,000		250,000,000	0
Interest rate on senior notes			3.875%
Unaccreted discount			$ 200,000				196,000		196,000	0
Interest rate yielded per annum upon issuance			3.885%
Senior Notes [Member] | 6.500% Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							0		0	299,308,000
Senior notes face value							$ 0		$ 0	300,000,000
Interest rate on senior notes							6.50%		6.50%
Unaccreted discount							$ 0		$ 0	692,000
Unamortized discount (premium)		$ 1,200,000
Debt redemption price		333,200,000
Notes and bonds payable		300,000,000
Debt redemption, accrued interest		6,400,000
Make whole payment		26,800,000
Unaccreted discounts and amortization costs		$ (28,000,000)
Mortgage notes payable [Member] | 3.875% Unsecured Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Notes and bonds payable							$ 249,804,000		$ 249,804,000	0
Interest rate on senior notes							3.88%		3.88%
Mortgage notes payable [Member] | 4.15%-7.63% Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable							$ 129,087,000		$ 129,087,000	173,267,000
Senior notes face value							128,161,000		128,161,000	172,530,000
Unaccreted discount							$ 1,779,000		1,779,000	2,468,000
Amortization of debt discount									800,000	1,100,000	1,200,000
Amortization of debt premium									$ 1,000,000	$ 1,000,000	$ 700,000
LIBOR [Member] | Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member]
Debt Instrument [Line Items]
Basis spread over LIBOR on outstanding balance (percent)riable Rate									1.15%	1.40%
Contractual interest rates, basis spread on variable rate									1.20%
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR									1.15%
Basis spread over LIBOR on outstanding balance (percent)riable Rate									1.20%	1.45%
Contractual interest rates, basis spread on variable rate									1.15%
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR				0.95%
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR				1.75%
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Loss on forward starting interest rate swap agreements recognized in OCI			$ 1,700,000						$ (1,684,000)

[1] Includes discount accretion and premium amortization related to the Company’s Senior Notes due 2021, Senior Notes due 2023, Senior Notes due 2025, and 16 mortgage notes payable.