Notes and Bonds Payable - Narrative (Details) (USD $)	1 Months Ended	9 Months Ended	1 Months Ended	0 Months Ended	12 Months Ended					1 Months Ended
	Feb. 15, 2013	Sep. 30, 2014	Feb. 28, 2014	Feb. 15, 2013 extension_option	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Feb. 17, 2015	Oct. 14, 2011 Lender	Dec. 13, 2010	Mar. 26, 2013	Dec. 04, 2009
Debt Instrument [Line Items]
Notes and bonds payable					$ 1,403,692,000	[1]	$ 1,348,459,000	[1]
Unamortized discount (premium)					3,838,000	[2]
Unsecured Credit Facility due 2017 [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Anticipated 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		0
Long-term Debt, Weighted Average Interest Rate					1.60%
Senior notes face value			200,000,000.0
Number of Lenders Term Loan			9
Contractual interest rates, basis spread on variable rate		1.45%
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					85,000,000		238,000,000
Unsecured credit facility											700,000,000.0
Number of Lenders of unsecured credit facility											17
Number of extension options				2
Period of extension option				6 months
Extension fees as percentage of aggregate commitments				0.075%
Facility fee on the aggregate amount of commitments				0.30%
Amount outstanding under Unsecured Credit Facility					85,000,000
Weighted average interest rate (LOC)					1.60%
Remaining borrowing capacity					615,000,000
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					397,864,000		397,578,000
Senior notes face value					400,000,000		400,000,000					400,000,000.0
Interest rate on senior notes					5.75%							5.75%
Unaccreted discount					2,136,000		2,422,000
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
Senior Notes [Member] | 3.75% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Notes and bonds payable					248,253,000		248,077,000
Senior notes face value					250,000,000		250,000,000						250,000,000.0
Interest rate on senior notes					3.75%								3.75%
Unaccreted discount					1,747,000		1,923,000						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		100,000
Senior Notes [Member] | 6.500% Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Notes and bonds payable					299,308,000		299,008,000
Senior notes face value					300,000,000		300,000,000							300,000,000.0
Interest rate on senior notes					6.50%									6.50%
Unaccreted discount					692,000		992,000
Unamortized discount (premium)														2,000,000
Authorized Repurchase Of Debt Amount					50,000,000
Interest rate yielded per annum upon issuance														6.618%
Amount of amortization included in interest expense on the Company's consolidated Statements of operations					300,000
Mortgage notes payable [Member] | 5.000%-7.63% Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable					173,267,000		165,796,000
Senior notes face value					172,530,000		166,684,000
Unaccreted discount					2,468,000		3,596,000
Amortization of debt discount					1,100,000		1,200,000		1,000,000
Amortization of debt premium					1,000,000		700,000		700,000
LIBOR [Member] | Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate					1.45%
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR				1.40%
Contractual interest rates, basis spread on variable rate					1.40%
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%
Subsequent Event [Member] | Senior Notes [Member] | 5.45% Secured Note [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount										15,000,000
Interest rate on senior notes										5.45%
Accrued interest as of redemption date										$ 100,000

[1] Level 3 - Fair value derived from valuation techniques in which one of more significant inputs or significant value drivers are unobservable.

[2] Includes discount accretion and premium amortization related to the Company’s Senior Notes due 2017, Senior Notes due 2021, Senior Notes due 2023, and 17 mortgage notes payable.