Notes Payable (Narrative) (Details) $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
		May 31, 2018	Mar. 31, 2019 USD ($) yr	Dec. 31, 2018 USD ($)	May 12, 2018 USD ($)	Mar. 31, 2018 USD ($)
Notes Payable			$ 2,124,800	$ 2,321,600
Unamortized debt discounts and debt issuance costs			13,100	13,900
Unsecured Debt			2,079,136	1,836,427
Available amount under unsecured credit facility			$ 647,900
Weighted Average Interest Rate			3.90%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr			5.6
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility			$ 50,000
Outstanding balance under credit facility			10,100
Short-term Debt [Member]
Maximum borrowing capacity under unsecured credit facility					$ 45,000
Debt Instrument, Basis Spread on Variable Rate		0.95%
Other Short-term Borrowings			0
Available amount under unsecured credit facility			45,000
Commercial Banks [Member] | Unsecured Credit Facility [Member]
Unsecured Debt			242,000	0
Commercial Banks [Member] | Short-term Debt [Member]
Unsecured Debt			341,600	99,600
Senior Unsecured Notes [Member]
Unsecured Debt	[1]		1,737,500	$ 1,736,800
Unsecured Credit Facility [Member]
Unsecured Credit Facility, Current Borrowing Capacity			$ 600,000
Unsecured Credit Facility, Expiration Date			Mar. 08, 2023	Aug. 07, 2019
Maximum borrowing capacity under unsecured credit facility			$ 900,000
Maximum term of bid rate loans (days)			180
Lesser of amount stated or the amount available under the unsecured credit facility			$ 450,000
Floating rate notes payable [Member]
Notes Payable			$ 341,600	$ 99,600		$ 175,000
Weighted Average Interest Rate			3.30%			2.20%
Secured Notes Fixed Rate Debt due 2019 [Member] | Secured Debt [Member]
Repayments of Secured Debt			$ 439,300

[1] Unamortized debt discounts and debt issuance costs of $13.1 million and $13.9 million are included in senior unsecured and secured notes payable as of March 31, 2019 and December 31, 2018, respectively.