Notes Payable (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2020 USD ($) instrument	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) instrument	Sep. 30, 2017 USD ($) instrument	Dec. 31, 2016 USD ($) instrument	Oct. 31, 2015 USD ($) instrument	May 31, 2014 USD ($) instrument	Apr. 30, 2013 USD ($) instrument
Debt Instrument [Line Items]
Loss deferred in other comprehensive income		$ 0	$ 3,920,000	$ 7,617,000	$ 3,920,000
Proceeds from notes payable				692,646,000	0
Payment of debt issuance costs				7,809,000	115,000
Payment of make-whole premium				$ 16,679,000	$ 0
Forward Swap
Debt Instrument [Line Items]
Number of interest rate derivatives terminated | instrument	3					2	2	2	4	3	4
Aggregate notional amount	$ 200,000,000					$ 250,000,000	$ 250,000,000	$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Loss deferred in other comprehensive income	13,141,000
2030 Notes | Loans Payable
Debt Instrument [Line Items]
Principal amount	$ 400,000,000
Stated interest rate	2.50%
Aggregate purchase price	$ 398,712,000
Discount	$ 1,288,000
Effective Rate	2.536%
Proceeds from notes payable	$ 395,062,000
Payment of debt issuance costs	3,650,000
2050 Notes | Loans Payable
Debt Instrument [Line Items]
Principal amount	$ 300,000,000
Stated interest rate	3.10%
Aggregate purchase price	$ 293,934,000
Discount	$ 6,066,000
Effective Rate	3.205%
Proceeds from notes payable	$ 290,459,000
Payment of debt issuance costs	$ 3,475,000
2022 Notes | Loans Payable
Debt Instrument [Line Items]
Stated interest rate	3.80%
Notes payable redeemed	$ 325,000,000
Redemption price, percent of principal amount	100.00%
Payment of make-whole premium	$ 16,679,000